diff --git a/.eslintrc b/.eslintrc
index 6152540354cf..8b8c53e5eef5 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -46,6 +46,7 @@
     "no-useless-call": 2,
     "no-useless-concat": 2,
     "no-warning-comments": [2, { "terms": ["do not submit"], "location": "anywhere" }],
+    "prefer-const": 2,
     "radix": 2,
     "semi": 2,
     "space-after-keywords": 2,
diff --git a/build-system/tasks/lint.js b/build-system/tasks/lint.js
index c59b2c2a3416..f4f9b406f55d 100644
--- a/build-system/tasks/lint.js
+++ b/build-system/tasks/lint.js
@@ -53,7 +53,7 @@ function lint() {
   }
 
   return stream.pipe(eslint(options))
-    .pipe(eslint.formatEach('compact', function(msg) {
+    .pipe(eslint.formatEach('stylish', function(msg) {
       errorsFound = true;
       util.log(util.colors.red(msg));
     }))
diff --git a/builtins/amp-ad.js b/builtins/amp-ad.js
index a784d5c70e02..c18fec6f1d90 100644
--- a/builtins/amp-ad.js
+++ b/builtins/amp-ad.js
@@ -63,15 +63,15 @@ const POSITION_FIXED_TAG_WHITELIST = {
  */
 export function scoreDimensions_(dims, maxWidth, maxHeight) {
   return dims.map(function(dim) {
-    let width = dim[0];
-    let height = dim[1];
-    let widthScore = Math.abs(width - maxWidth);
+    const width = dim[0];
+    const height = dim[1];
+    const widthScore = Math.abs(width - maxWidth);
     // if the width is over the max then we need to penalize it
-    let widthPenalty = Math.abs((maxWidth - width) * 3);
+    const widthPenalty = Math.abs((maxWidth - width) * 3);
     // we add a multiplier to height as we prioritize it more than width
-    let heightScore = Math.abs(height - maxHeight) * 2;
+    const heightScore = Math.abs(height - maxHeight) * 2;
     // if the height is over the max then we need to penalize it
-    let heightPenalty = Math.abs((maxHeight - height) * 2.5);
+    const heightPenalty = Math.abs((maxHeight - height) * 2.5);
 
     return (widthScore - widthPenalty) + (heightScore - heightPenalty);
   });
@@ -84,7 +84,7 @@ export function scoreDimensions_(dims, maxWidth, maxHeight) {
  */
 export function upgradeImages_(images) {
   Object.keys(images).forEach(key => {
-    let curDimImgs = images[key];
+    const curDimImgs = images[key];
     curDimImgs.forEach((item, index) => {
       curDimImgs[index] = item.replace(/@1x\.png$/, '@2x.png');
     });
@@ -148,9 +148,9 @@ export function installAd(win) {
     prefetchAd_() {
       // We always need the bootstrap.
       prefetchBootstrap(this.getWin());
-      let type = this.element.getAttribute('type');
-      let prefetch = adPrefetch[type];
-      let preconnect = adPreconnect[type];
+      const type = this.element.getAttribute('type');
+      const prefetch = adPrefetch[type];
+      const preconnect = adPreconnect[type];
       if (typeof prefetch == 'string') {
         this.preconnect.prefetch(prefetch);
       } else if (prefetch) {
@@ -166,7 +166,7 @@ export function installAd(win) {
         });
       }
       // If fully qualified src for ad script is specified we prefetch that.
-      let src = this.element.getAttribute('src');
+      const src = this.element.getAttribute('src');
       if (src) {
         this.preconnect.prefetch(src);
       }
@@ -187,7 +187,7 @@ export function installAd(win) {
      */
     isPositionFixed() {
       let el = this.element;
-      let body = el.ownerDocument.body;
+      const body = el.ownerDocument.body;
       do {
         if (POSITION_FIXED_TAG_WHITELIST[el.tagName]) {
           return false;
@@ -248,7 +248,7 @@ export function installAd(win) {
         margin: 'auto',
       });
 
-      let winner = this.getPlaceholderImage_();
+      const winner = this.getPlaceholderImage_();
       img.src = `https://ampproject.org/backfill/${winner}`;
       this.placeholder_ = a;
       a.appendChild(img);
@@ -261,12 +261,12 @@ export function installAd(win) {
      * @return {string} The image URL.
      */
     getPlaceholderImage_() {
-      let scores = scoreDimensions_(BACKFILL_DIMENSIONS_,
+      const scores = scoreDimensions_(BACKFILL_DIMENSIONS_,
           this.element./*REVIEW*/clientWidth,
           this.element./*REVIEW*/clientHeight);
-      let dims = BACKFILL_DIMENSIONS_[
+      const dims = BACKFILL_DIMENSIONS_[
           scores.indexOf(Math.max.apply(Math, scores))];
-      let images = BACKFILL_IMGS_[dims.join('x')];
+      const images = BACKFILL_IMGS_[dims.join('x')];
       // do we need a more sophisticated randomizer?
       return images[Math.floor(Math.random() * images.length)];
     }
diff --git a/builtins/amp-img.js b/builtins/amp-img.js
index 423970a78f25..9bb0a62dc52c 100644
--- a/builtins/amp-img.js
+++ b/builtins/amp-img.js
@@ -82,7 +82,7 @@ export function installImg(win) {
       if (this.getLayoutWidth() <= 0) {
         return Promise.resolve();
       }
-      let src = this.srcset_.select(this.getLayoutWidth(), this.getDpr()).url;
+      const src = this.srcset_.select(this.getLayoutWidth(), this.getDpr()).url;
       if (src == this.img_.getAttribute('src')) {
         return Promise.resolve();
       }
diff --git a/builtins/amp-pixel.js b/builtins/amp-pixel.js
index f619d345e54e..6178035dc0be 100644
--- a/builtins/amp-pixel.js
+++ b/builtins/amp-pixel.js
@@ -51,7 +51,7 @@ export function installPixel(win) {
      * Returns the host of the canonical URL for this AMP document.
      */
     'CANONICAL_HOST': () => {
-      let url = parseUrl(documentInfoFor(win).canonicalUrl);
+      const url = parseUrl(documentInfoFor(win).canonicalUrl);
       return url && url.hostname;
     },
 
@@ -59,7 +59,7 @@ export function installPixel(win) {
      * Returns the path of the canonical URL for this AMP document.
      */
     'CANONICAL_PATH': () => {
-      let url = parseUrl(documentInfoFor(win).canonicalUrl);
+      const url = parseUrl(documentInfoFor(win).canonicalUrl);
       return url && url.pathname;
     },
 
@@ -88,7 +88,7 @@ export function installPixel(win) {
      * Returns the host of the URL for this AMP document.
      */
     'AMPDOC_HOST': () => {
-      let url = parseUrl(win.location.href);
+      const url = parseUrl(win.location.href);
       return url && url.hostname;
     }
   };
@@ -98,7 +98,7 @@ export function installPixel(win) {
    */
   const REPLACEMENT_EXPR = (() => {
     let all = '';
-    for (let k in REPLACEMENTS) {
+    for (const k in REPLACEMENTS) {
       all += (all.length > 0 ? '|' : '') + k;
     }
     return new RegExp('\\$(' + all + ')', 'g');
diff --git a/builtins/amp-video.js b/builtins/amp-video.js
index 73d69c307641..6f72373e27a7 100644
--- a/builtins/amp-video.js
+++ b/builtins/amp-video.js
@@ -37,9 +37,9 @@ export function installVideo(win) {
 
     /** @override */
     layoutCallback() {
-      let width = this.element.getAttribute('width');
-      let height = this.element.getAttribute('height');
-      let video = document.createElement('video');
+      const width = this.element.getAttribute('width');
+      const height = this.element.getAttribute('height');
+      const video = document.createElement('video');
       if (!video.play) {
         this.toggleFallback(true);
         return Promise.resolve();
diff --git a/extensions/amp-anim/0.1/amp-anim.js b/extensions/amp-anim/0.1/amp-anim.js
index 97771c836d8e..ab97c3d884ad 100644
--- a/extensions/amp-anim/0.1/amp-anim.js
+++ b/extensions/amp-anim/0.1/amp-anim.js
@@ -86,7 +86,7 @@ class AmpAnim extends AMP.BaseElement {
 
   /** @private */
   updateInViewport_() {
-    let inViewport = this.isInViewport();
+    const inViewport = this.isInViewport();
     this.togglePlaceholder(!inViewport);
     st.toggle(this.img_, inViewport);
   }
@@ -99,7 +99,7 @@ class AmpAnim extends AMP.BaseElement {
     if (this.getLayoutWidth() <= 0) {
       return Promise.resolve();
     }
-    let src = this.srcset_.select(this.getLayoutWidth(),
+    const src = this.srcset_.select(this.getLayoutWidth(),
         this.getDpr()).url;
     if (src == this.img_.getAttribute('src')) {
       return Promise.resolve();
diff --git a/extensions/amp-audio/0.1/amp-audio.js b/extensions/amp-audio/0.1/amp-audio.js
index 1e2648346912..2c3d798f0810 100644
--- a/extensions/amp-audio/0.1/amp-audio.js
+++ b/extensions/amp-audio/0.1/amp-audio.js
@@ -31,7 +31,7 @@ export class AmpAudio extends AMP.BaseElement {
 
   /** @override */
   layoutCallback() {
-    let audio = document.createElement('audio');
+    const audio = document.createElement('audio');
     if (!audio.play) {
       this.toggleFallback(true);
       return Promise.resolve();
diff --git a/extensions/amp-audio/0.1/test/test-amp-audio.js b/extensions/amp-audio/0.1/test/test-amp-audio.js
index 3db11e76a6ab..c35df92eddb4 100644
--- a/extensions/amp-audio/0.1/test/test-amp-audio.js
+++ b/extensions/amp-audio/0.1/test/test-amp-audio.js
@@ -53,7 +53,7 @@ describe('amp-audio', () => {
           child = iframe.doc.createTextNode(childNodeAttrs.text);
         } else {
           child = iframe.doc.createElement(childNodeAttrs.tag);
-          for (let key in childNodeAttrs) {
+          for (const key in childNodeAttrs) {
             if (key !== 'tag') {
               child.setAttribute(key, childNodeAttrs[key]);
             }
@@ -148,17 +148,17 @@ describe('amp-audio', () => {
   });
 
   it('should fallback when not available', () => {
-    let savedCreateElement = document.createElement;
+    const savedCreateElement = document.createElement;
     document.createElement = name => {
       if (name == 'audio') {
         return savedCreateElement.call(document, 'audio2');
       }
       return savedCreateElement.call(document, name);
     };
-    let element = document.createElement('div');
+    const element = document.createElement('div');
     element.toggleFallback = sinon.spy();
-    let audio = new AmpAudio(element);
-    let promise = audio.layoutCallback();
+    const audio = new AmpAudio(element);
+    const promise = audio.layoutCallback();
     document.createElement = savedCreateElement;
     return promise.then(() => {
       expect(audio.audio_).to.be.undefined;
diff --git a/extensions/amp-carousel/0.1/carousel.js b/extensions/amp-carousel/0.1/carousel.js
index 0d9c13b7db15..ecc87ae7327d 100644
--- a/extensions/amp-carousel/0.1/carousel.js
+++ b/extensions/amp-carousel/0.1/carousel.js
@@ -115,9 +115,9 @@ export class AmpCarousel extends BaseCarousel {
    * @private
    */
   nextPos_(pos, dir) {
-    let containerWidth = this.element./*OK*/offsetWidth;
-    let fullWidth = this.container_./*OK*/scrollWidth;
-    let newPos = pos + dir * containerWidth;
+    const containerWidth = this.element./*OK*/offsetWidth;
+    const fullWidth = this.container_./*OK*/scrollWidth;
+    const newPos = pos + dir * containerWidth;
     if (newPos < 0) {
       return 0;
     }
@@ -134,9 +134,9 @@ export class AmpCarousel extends BaseCarousel {
    * @private
    */
   withinWindow_(pos, callback) {
-    let containerWidth = this.getLayoutWidth();
+    const containerWidth = this.getLayoutWidth();
     for (let i = 0; i < this.cells_.length; i++) {
-      let cell = this.cells_[i];
+      const cell = this.cells_[i];
       if (cell./*OK*/offsetLeft + cell./*OK*/offsetWidth >= pos &&
             cell./*OK*/offsetLeft <= pos + containerWidth) {
         callback(cell);
@@ -174,7 +174,7 @@ export class AmpCarousel extends BaseCarousel {
    * @private
    */
   updateInViewport_(newPos, oldPos) {
-    let seen = [];
+    const seen = [];
     this.withinWindow_(newPos, cell => {
       seen.push(cell);
       this.updateInViewport(cell, true);
@@ -201,7 +201,7 @@ export class AmpCarousel extends BaseCarousel {
     /** @private {?Motion} */
     this.motion_ = null;
 
-    let gestures = Gestures.get(this.element);
+    const gestures = Gestures.get(this.element);
     gestures.onGesture(SwipeXRecognizer, e => {
       if (e.data.first) {
         this.onSwipeStart_(e.data);
@@ -253,7 +253,7 @@ export class AmpCarousel extends BaseCarousel {
     if (Math.abs(swipe.velocityX) > 0.1) {
       this.motion_ = continueMotion(this.pos_, 0, -swipe.velocityX, 0,
           (x, y) => {
-            let newPos = (this.boundPos_(x, true) +
+            const newPos = (this.boundPos_(x, true) +
                 this.boundPos_(x, false)) * 0.5;
             if (Math.abs(newPos - this.pos_) <= 1) {
               // Hit the wall: stop motion.
@@ -270,11 +270,11 @@ export class AmpCarousel extends BaseCarousel {
       promise = Promise.resolve();
     }
     return promise.then(() => {
-      let newPos = this.boundPos_(this.pos_, false);
+      const newPos = this.boundPos_(this.pos_, false);
       if (Math.abs(newPos - this.pos_) < 1) {
         return undefined;
       }
-      let posFunc = tr.numeric(this.pos_, newPos);
+      const posFunc = tr.numeric(this.pos_, newPos);
       return Animation.animate(time => {
         this.pos_ = posFunc(time);
         st.setStyles(this.container_, {
@@ -290,8 +290,8 @@ export class AmpCarousel extends BaseCarousel {
 
   /** @private */
   updateBounds_() {
-    let containerWidth = this.element./*OK*/offsetWidth;
-    let scrollWidth = this.container_./*OK*/scrollWidth;
+    const containerWidth = this.element./*OK*/offsetWidth;
+    const scrollWidth = this.container_./*OK*/scrollWidth;
     this.minPos_ = 0;
     this.maxPos_ = Math.max(scrollWidth - containerWidth, 0);
     this.extent_ = Math.min(containerWidth * 0.4, 200);
@@ -304,7 +304,7 @@ export class AmpCarousel extends BaseCarousel {
    * @private
    */
   boundPos_(pos, allowExtent) {
-    let extent = allowExtent ? this.extent_ : 0;
+    const extent = allowExtent ? this.extent_ : 0;
     return Math.min(this.maxPos_ + extent,
         Math.max(this.minPos_ - extent, pos));
   }
@@ -316,9 +316,9 @@ export class AmpCarousel extends BaseCarousel {
 
   /** @override */
   hasNext() {
-    let containerWidth = this.getLayoutWidth();
-    let scrollWidth = this.container_./*OK*/scrollWidth;
-    let maxPos = Math.max(scrollWidth - containerWidth, 0);
+    const containerWidth = this.getLayoutWidth();
+    const scrollWidth = this.container_./*OK*/scrollWidth;
+    const maxPos = Math.max(scrollWidth - containerWidth, 0);
     return this.pos_ != maxPos;
   }
 }
diff --git a/extensions/amp-carousel/0.1/slides.js b/extensions/amp-carousel/0.1/slides.js
index c6b3590e848d..170c8e5bd133 100644
--- a/extensions/amp-carousel/0.1/slides.js
+++ b/extensions/amp-carousel/0.1/slides.js
@@ -106,7 +106,7 @@ export class AmpSlides extends BaseCarousel {
       return;
     }
 
-    let autoplayValue = Number(this.element.getAttribute('autoplay'));
+    const autoplayValue = Number(this.element.getAttribute('autoplay'));
     // If it isn't a number and is not greater than 0 then don't assign
     // and use the default.
     if (autoplayValue > 0) {
@@ -253,7 +253,7 @@ export class AmpSlides extends BaseCarousel {
      * }} */
     this.swipeState_ = null;
 
-    let gestures = Gestures.get(this.element);
+    const gestures = Gestures.get(this.element);
     gestures.onGesture(SwipeXRecognizer, e => {
       if (e.data.first) {
         this.onSwipeStart_(e.data);
@@ -270,25 +270,25 @@ export class AmpSlides extends BaseCarousel {
    * @private
    */
   onSwipeStart_(swipe) {
-    let currentSlide = this.slides_[this.currentIndex_];
-    let containerWidth = this.element./*OK*/offsetWidth;
+    const currentSlide = this.slides_[this.currentIndex_];
+    const containerWidth = this.element./*OK*/offsetWidth;
     let minDelta = 0;
     let maxDelta = 0;
     let prevTr = tr.NOOP;
     let nextTr = tr.NOOP;
-    let prevIndex = AmpSlides.getRelativeIndex(this.currentIndex_,
+    const prevIndex = AmpSlides.getRelativeIndex(this.currentIndex_,
         -1, this.slides_.length);
-    let nextIndex = AmpSlides.getRelativeIndex(this.currentIndex_,
+    const nextIndex = AmpSlides.getRelativeIndex(this.currentIndex_,
         1, this.slides_.length);
 
     if (this.isLooping_ || this.currentIndex_ - 1 >= 0) {
-      let prevSlide = this.slides_[prevIndex];
+      const prevSlide = this.slides_[prevIndex];
       this.prepareSlide_(prevSlide, -1);
       prevTr = this.createTransition_(currentSlide, prevSlide, -1);
       minDelta = -1;
     }
     if (this.isLooping_ || this.currentIndex_ + 1 < this.slides_.length) {
-      let nextSlide = this.slides_[nextIndex];
+      const nextSlide = this.slides_[nextIndex];
       this.prepareSlide_(nextSlide, 1);
       nextTr = this.createTransition_(currentSlide, nextSlide, 1);
       maxDelta = 1;
@@ -311,7 +311,7 @@ export class AmpSlides extends BaseCarousel {
    * @private
    */
   onSwipe_(swipe) {
-    let s = this.swipeState_;
+    const s = this.swipeState_;
     if (!s || s.currentIndex != this.currentIndex_) {
       return;
     }
@@ -319,7 +319,7 @@ export class AmpSlides extends BaseCarousel {
     // Translate the gesture position to be a number between -1 and 1,
     // with negative values indiamping sliding to the previous slide and
     // positive indiamping sliding to the next slide.
-    let pos = Math.min(s.max, Math.max(s.min,
+    const pos = Math.min(s.max, Math.max(s.min,
         -swipe.deltaX / s.containerWidth));
 
     s.nextTr(pos > 0 ? pos : 0);
@@ -333,7 +333,7 @@ export class AmpSlides extends BaseCarousel {
    * @private
    */
   onSwipeEnd_(swipe) {
-    let s = this.swipeState_;
+    const s = this.swipeState_;
     if (!s || s.currentIndex != this.currentIndex_) {
       return;
     }
@@ -345,12 +345,12 @@ export class AmpSlides extends BaseCarousel {
           (Math.abs(swipe.velocityX) > 0.2 ? 1 : 0);
     }
     advPos = Math.min(s.max, Math.max(s.min, advPos));
-    let newPos = Math.abs(advPos) >= 0.55 ? Math.sign(advPos) : 0;
+    const newPos = Math.abs(advPos) >= 0.55 ? Math.sign(advPos) : 0;
     let promise;
     if (newPos != s.pos) {
-      let posFunc = tr.numeric(s.pos, newPos);
+      const posFunc = tr.numeric(s.pos, newPos);
       promise = Animation.animate(time => {
-        let pos = posFunc(time);
+        const pos = posFunc(time);
         s.nextTr(pos > 0 ? pos : 0);
         s.prevTr(pos < 0 ? -pos : 0);
         s.pos = pos;
@@ -362,7 +362,7 @@ export class AmpSlides extends BaseCarousel {
       if (s.currentIndex != this.currentIndex_) {
         return;
       }
-      let oldSlide = this.slides_[this.currentIndex_];
+      const oldSlide = this.slides_[this.currentIndex_];
       if (newPos > 0.5) {
         s.nextTr(1);
         this.currentIndex_ = s.nextIndex;
diff --git a/extensions/amp-carousel/0.1/test/test-carousel.js b/extensions/amp-carousel/0.1/test/test-carousel.js
index f214a1b0b911..cb69ad2d6c29 100644
--- a/extensions/amp-carousel/0.1/test/test-carousel.js
+++ b/extensions/amp-carousel/0.1/test/test-carousel.js
@@ -97,13 +97,13 @@ describe('Carousel gestures', () => {
 
   it('should continue innertia', () => {
     carousel.onSwipeStart_({});
-    let promise = carousel.onSwipeEnd_({deltaX: 0, velocityX: -0.11});
+    const promise = carousel.onSwipeEnd_({deltaX: 0, velocityX: -0.11});
     expect(carousel.motion_).to.not.equal(null);
   });
 
   it('should not continue innertia', () => {
     carousel.onSwipeStart_({});
-    let promise = carousel.onSwipeEnd_({deltaX: 0, velocityX: -0.01});
+    const promise = carousel.onSwipeEnd_({deltaX: 0, velocityX: -0.01});
     expect(carousel.motion_).to.equal(null);
   });
 });
diff --git a/extensions/amp-carousel/0.1/test/test-slides.js b/extensions/amp-carousel/0.1/test/test-slides.js
index 07a819de8d46..49cacc3bb0a0 100644
--- a/extensions/amp-carousel/0.1/test/test-slides.js
+++ b/extensions/amp-carousel/0.1/test/test-slides.js
@@ -150,8 +150,8 @@ describe('Slides functional', () => {
     });
 
     it('should update on swipe within range in neg direction', () => {
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
       slides.swipeState_ = {
         currentIndex: 0,
@@ -173,8 +173,8 @@ describe('Slides functional', () => {
     });
 
     it('should update on swipe within range in pos direction', () => {
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 1;
       slides.swipeState_ = {
         currentIndex: 1,
@@ -196,8 +196,8 @@ describe('Slides functional', () => {
     });
 
     it('should stay in-bounds on swipe', () => {
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
       slides.swipeState_ = {
         currentIndex: 0,
@@ -235,10 +235,10 @@ describe('Slides functional', () => {
     });
 
     it.skip('should go next after threshold', () => {
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
-      let s = {
+      const s = {
         currentIndex: 0,
         prevIndex: 2,
         nextIndex: 1,
@@ -250,7 +250,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: 0});
+      const promise = slides.onSwipeEnd_({velocityX: 0});
       expect(slides.swipeState_).to.equal(null);
       return promise.then(() => {
         expect(nextTr.callCount).to.be.gt(1);
@@ -266,10 +266,10 @@ describe('Slides functional', () => {
     it('should not go past first item with a negative value when not ' +
        ' looping', () => {
       slides.isLooping_ = true;
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
-      let s = {
+      const s = {
         currentIndex: 0,
         prevIndex: 2,
         nextIndex: 1,
@@ -281,7 +281,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: 0});
+      const promise = slides.onSwipeEnd_({velocityX: 0});
       return promise.then(() => {
         expect(slides.currentIndex_).to.equal(0);
         expect(switchCallback.callCount).to.equal(0);
@@ -290,10 +290,10 @@ describe('Slides functional', () => {
 
     it('should go past first item with a negative value when looping', () => {
       slides.isLooping_ = true;
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
-      let s = {
+      const s = {
         currentIndex: 0,
         prevIndex: 2,
         nextIndex: 1,
@@ -305,7 +305,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: 0});
+      const promise = slides.onSwipeEnd_({velocityX: 0});
       return promise.then(() => {
         expect(slides.currentIndex_).to.equal(2);
         expect(switchCallback.firstCall.args[0]).to.equal(slide0);
@@ -316,10 +316,10 @@ describe('Slides functional', () => {
     it('should not go past last item with a positive value when ' +
        'not looping', () => {
       slides.isLooping_ = true;
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 2;
-      let s = {
+      const s = {
         currentIndex: 2,
         prevIndex: 1,
         nextIndex: 0,
@@ -331,7 +331,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: 0});
+      const promise = slides.onSwipeEnd_({velocityX: 0});
       return promise.then(() => {
         expect(slides.currentIndex_).to.equal(2);
         expect(switchCallback.callCount).to.equal(0);
@@ -340,10 +340,10 @@ describe('Slides functional', () => {
 
     it('should go past last item with a positive value when looping', () => {
       slides.isLooping_ = true;
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 2;
-      let s = {
+      const s = {
         currentIndex: 2,
         prevIndex: 1,
         nextIndex: 0,
@@ -355,7 +355,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: 0});
+      const promise = slides.onSwipeEnd_({velocityX: 0});
       return promise.then(() => {
         expect(slides.currentIndex_).to.equal(0);
         expect(switchCallback.firstCall.args[0]).to.equal(slide2);
@@ -364,10 +364,10 @@ describe('Slides functional', () => {
     });
 
     it('should go next before threshold but with velocity', () => {
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
-      let s = {
+      const s = {
         currentIndex: 0,
         prevIndex: 2,
         nextIndex: 1,
@@ -379,7 +379,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: -0.5});
+      const promise = slides.onSwipeEnd_({velocityX: -0.5});
       expect(slides.swipeState_).to.equal(null);
       return promise.then(() => {
         expect(nextTr.callCount).to.be.gt(1);
@@ -393,10 +393,10 @@ describe('Slides functional', () => {
     });
 
     it('should bounce back before threshold and no velocity', () => {
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
-      let s = {
+      const s = {
         currentIndex: 0,
         prevIndex: 2,
         nextIndex: 1,
@@ -408,7 +408,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: 0});
+      const promise = slides.onSwipeEnd_({velocityX: 0});
       expect(slides.swipeState_).to.equal(null);
       return promise.then(() => {
         expect(nextTr.callCount).to.be.gt(1);
@@ -420,10 +420,10 @@ describe('Slides functional', () => {
     });
 
     it('should bounce back before threshold and opposite velocity', () => {
-      let prevTr = sinon.spy();
-      let nextTr = sinon.spy();
+      const prevTr = sinon.spy();
+      const nextTr = sinon.spy();
       slides.currentIndex_ = 0;
-      let s = {
+      const s = {
         currentIndex: 0,
         prevIndex: 2,
         nextIndex: 1,
@@ -435,7 +435,7 @@ describe('Slides functional', () => {
         nextTr: nextTr
       };
       slides.swipeState_ = s;
-      let promise = slides.onSwipeEnd_({velocityX: 0.5});
+      const promise = slides.onSwipeEnd_({velocityX: 0.5});
       expect(slides.swipeState_).to.equal(null);
       return promise.then(() => {
         expect(nextTr.callCount).to.be.gt(1);
diff --git a/extensions/amp-fit-text/0.1/amp-fit-text.js b/extensions/amp-fit-text/0.1/amp-fit-text.js
index 8ad69286886d..2828d5850272 100644
--- a/extensions/amp-fit-text/0.1/amp-fit-text.js
+++ b/extensions/amp-fit-text/0.1/amp-fit-text.js
@@ -96,8 +96,8 @@ class AmpFitText extends AMP.BaseElement {
 
   /** @private */
   updateFontSize_() {
-    let maxHeight = this.element./*OK*/offsetHeight;
-    let fontSize = calculateFontSize_(this.measurer_, maxHeight,
+    const maxHeight = this.element./*OK*/offsetHeight;
+    const fontSize = calculateFontSize_(this.measurer_, maxHeight,
         this.minFontSize_, this.maxFontSize_);
     this.contentWrapper_.style.fontSize = st.px(fontSize);
     updateOverflow_(this.contentWrapper_, this.measurer_, maxHeight,
@@ -119,9 +119,9 @@ export function calculateFontSize_(measurer, expectedHeight,
   maxFontSize++;
   // Binomial search for the best font size.
   while (maxFontSize - minFontSize > 1) {
-    let mid = Math.floor((minFontSize + maxFontSize) / 2);
+    const mid = Math.floor((minFontSize + maxFontSize) / 2);
     measurer.style.fontSize = st.px(mid);
-    let height = measurer./*OK*/offsetHeight;
+    const height = measurer./*OK*/offsetHeight;
     if (height > expectedHeight) {
       maxFontSize = mid;
     } else {
@@ -141,9 +141,9 @@ export function calculateFontSize_(measurer, expectedHeight,
  */
 export function updateOverflow_(content, measurer, maxHeight, fontSize) {
   measurer.style.fontSize = st.px(fontSize);
-  let overflown = measurer./*OK*/offsetHeight > maxHeight;
-  let lineHeight = fontSize * LINE_HEIGHT_EM_;
-  let numberOfLines = Math.floor(maxHeight / lineHeight);
+  const overflown = measurer./*OK*/offsetHeight > maxHeight;
+  const lineHeight = fontSize * LINE_HEIGHT_EM_;
+  const numberOfLines = Math.floor(maxHeight / lineHeight);
   content.classList.toggle('-amp-fit-text-content-overflown', overflown);
   st.setStyles(content, {
     lineClamp: overflown ? numberOfLines : '',
diff --git a/extensions/amp-fit-text/0.1/test/test-amp-fit-text.js b/extensions/amp-fit-text/0.1/test/test-amp-fit-text.js
index a3342a2fc155..276d8a6be926 100644
--- a/extensions/amp-fit-text/0.1/test/test-amp-fit-text.js
+++ b/extensions/amp-fit-text/0.1/test/test-amp-fit-text.js
@@ -135,7 +135,7 @@ describe('amp-fit-text updateOverflow', () => {
   });
 
   function getLineClamp() {
-    for (let k in content.style) {
+    for (const k in content.style) {
       if (k == 'lineClamp' || k.match(/.*LineClamp/)) {
         return content.style[k];
       }
diff --git a/extensions/amp-iframe/0.1/amp-iframe.js b/extensions/amp-iframe/0.1/amp-iframe.js
index 4c9e57a43c7f..4e74506bec21 100644
--- a/extensions/amp-iframe/0.1/amp-iframe.js
+++ b/extensions/amp-iframe/0.1/amp-iframe.js
@@ -154,8 +154,8 @@ class AmpIframe extends AMP.BaseElement {
         this.element.setAttribute('width', data.width);
       }
       if (data.height !== undefined) {
-        let newHeight = Math.max(this.element./*OK*/offsetHeight + data.height -
-            this.iframe_./*OK*/offsetHeight, data.height);
+        const newHeight = Math.max(this.element./*OK*/offsetHeight +
+            data.height - this.iframe_./*OK*/offsetHeight, data.height);
         iframe.height = data.height;
         this.element.setAttribute('height', newHeight);
         this.updateHeight_(newHeight);
@@ -223,8 +223,8 @@ function makeIOsScrollable(element, iframe) {
  */
 function listen(iframe, typeOfMessage, callback) {
   assert(iframe.src, 'only iframes with src supported');
-  let origin = parseUrl(iframe.src).origin;
-  let win = iframe.ownerDocument.defaultView;
+  const origin = parseUrl(iframe.src).origin;
+  const win = iframe.ownerDocument.defaultView;
   win.addEventListener('message', function(event) {
     if (event.origin != origin) {
       return;
diff --git a/extensions/amp-iframe/0.1/test/test-amp-iframe.js b/extensions/amp-iframe/0.1/test/test-amp-iframe.js
index dbb43a6462ab..8b66b8714ecd 100644
--- a/extensions/amp-iframe/0.1/test/test-amp-iframe.js
+++ b/extensions/amp-iframe/0.1/test/test-amp-iframe.js
@@ -56,7 +56,7 @@ describe('amp-iframe', () => {
         i.style.transform = 'translateY(' + opt_translateY + ')';
       }
       if (attributes.resizable !== undefined) {
-        let overflowEl = iframe.doc.createElement('div');
+        const overflowEl = iframe.doc.createElement('div');
         overflowEl.setAttribute('overflow', '');
         i.appendChild(overflowEl);
       }
@@ -266,9 +266,9 @@ describe('amp-iframe', () => {
       height: 100,
       resizable: ''
     }).then(amp => {
-      let impl = amp.container.implementation_;
+      const impl = amp.container.implementation_;
       impl.layoutCallback();
-      let p = new Promise((resolve, reject) => {
+      const p = new Promise((resolve, reject) => {
         impl.updateHeight_ = newHeight => {
           resolve({amp: amp, newHeight: newHeight});
         };
@@ -293,7 +293,7 @@ describe('amp-iframe', () => {
       height: 100,
       resizable: ''
     }).then(amp => {
-      let impl = amp.container.implementation_;
+      const impl = amp.container.implementation_;
       impl.requestChangeHeight = sinon.spy();
       impl.changeHeight = sinon.spy();
       impl.updateHeight_(217);
@@ -301,7 +301,7 @@ describe('amp-iframe', () => {
       expect(impl.requestChangeHeight.callCount).to.equal(1);
       expect(impl.requestChangeHeight.firstCall.args[0]).to.equal(217);
 
-      let fallback = impl.requestChangeHeight.firstCall.args[1];
+      const fallback = impl.requestChangeHeight.firstCall.args[1];
       fallback(219);
       expect(impl.overflowElement_).to.not.be.null;
       expect(impl.overflowElement_).to.have.class('-amp-overflow');
@@ -320,7 +320,7 @@ describe('amp-iframe', () => {
       width: 100,
       height: 100
     }).then(amp => {
-      let impl = amp.container.implementation_;
+      const impl = amp.container.implementation_;
       impl.requestChangeHeight = sinon.spy();
       impl.changeHeight = sinon.spy();
       impl.updateHeight_(217);
diff --git a/extensions/amp-image-lightbox/0.1/amp-image-lightbox.js b/extensions/amp-image-lightbox/0.1/amp-image-lightbox.js
index 6e4f1b60ec7e..bac95bb9eb55 100644
--- a/extensions/amp-image-lightbox/0.1/amp-image-lightbox.js
+++ b/extensions/amp-image-lightbox/0.1/amp-image-lightbox.js
@@ -222,7 +222,7 @@ export class ImageViewer {
     this.viewerBox_ = layoutRectFromDomRect(this.viewer_
         ./*OK*/getBoundingClientRect());
 
-    let sf = Math.min(this.viewerBox_.width / this.sourceWidth_,
+    const sf = Math.min(this.viewerBox_.width / this.sourceWidth_,
         this.viewerBox_.height / this.sourceHeight_);
     let width = Math.min(this.sourceWidth_ * sf, this.viewerBox_.width);
     let height = Math.min(this.sourceHeight_ * sf, this.viewerBox_.height);
@@ -264,8 +264,8 @@ export class ImageViewer {
    */
   updateSrc_() {
     this.maxSeenScale_ = Math.max(this.maxSeenScale_, this.scale_);
-    let width = this.imageBox_.width * this.maxSeenScale_;
-    let src = this.srcset_.select(width, this.lightbox_.getDpr()).url;
+    const width = this.imageBox_.width * this.maxSeenScale_;
+    const src = this.srcset_.select(width, this.lightbox_.getDpr()).url;
     if (src == this.image_.getAttribute('src')) {
       return Promise.resolve();
     }
@@ -280,7 +280,7 @@ export class ImageViewer {
 
   /** @private */
   setupGestures_() {
-    let gestures = Gestures.get(this.image_);
+    const gestures = Gestures.get(this.image_);
 
     // Toggle viewer mode.
     gestures.onGesture(TapRecognizer, () => {
@@ -308,8 +308,8 @@ export class ImageViewer {
       } else {
         newScale = this.minScale_;
       }
-      let deltaX = this.viewerBox_.width / 2 - e.data.clientX;
-      let deltaY = this.viewerBox_.height / 2 - e.data.clientY;
+      const deltaX = this.viewerBox_.width / 2 - e.data.clientX;
+      const deltaY = this.viewerBox_.height / 2 - e.data.clientY;
       this.onZoom_(newScale, deltaX, deltaY, true).then(() => {
         return this.onZoomRelease_(0, 0, 0, 0, 0, 0);
       });
@@ -383,7 +383,7 @@ export class ImageViewer {
   updatePanZoomBounds_(scale) {
     let maxY = 0;
     let minY = 0;
-    let dh = this.viewerBox_.height - this.imageBox_.height * scale;
+    const dh = this.viewerBox_.height - this.imageBox_.height * scale;
     if (dh >= 0) {
       minY = maxY = 0;
     } else {
@@ -393,7 +393,7 @@ export class ImageViewer {
 
     let maxX = 0;
     let minX = 0;
-    let dw = this.viewerBox_.width - this.imageBox_.width * scale;
+    const dw = this.viewerBox_.width - this.imageBox_.width * scale;
     if (dw >= 0) {
       minX = maxX = 0;
     } else {
@@ -429,8 +429,8 @@ export class ImageViewer {
    * @private
    */
   onMove_(deltaX, deltaY, animate) {
-    let newPosX = this.boundX_(this.startX_ + deltaX, true);
-    let newPosY = this.boundY_(this.startY_ + deltaY, true);
+    const newPosX = this.boundX_(this.startX_ + deltaX, true);
+    const newPosY = this.boundY_(this.startY_ + deltaY, true);
     this.set_(this.scale_, newPosX, newPosY, animate);
   }
 
@@ -442,7 +442,7 @@ export class ImageViewer {
    * @private
    */
   onMoveRelease_(veloX, veloY) {
-    let deltaY = this.posY_ - this.startY_;
+    const deltaY = this.posY_ - this.startY_;
     if (this.scale_ == 1 && Math.abs(deltaY) > 10) {
       this.lightbox_.close();
       return;
@@ -451,8 +451,8 @@ export class ImageViewer {
     // Continue motion.
     this.motion_ = continueMotion(this.posX_, this.posY_, veloX, veloY,
         (x, y) => {
-          let newPosX = this.boundX_(x, true);
-          let newPosY = this.boundY_(y, true);
+          const newPosX = this.boundX_(x, true);
+          const newPosY = this.boundY_(y, true);
           if (Math.abs(newPosX - this.posX_) < 1 &&
                 Math.abs(newPosY - this.posY_) < 1) {
             // Hit the wall: stop motion.
@@ -478,17 +478,17 @@ export class ImageViewer {
    * @private
    */
   onZoomInc_(centerClientX, centerClientY, deltaX, deltaY) {
-    let dist = Math.sqrt(deltaX * deltaX + deltaY * deltaY);
+    const dist = Math.sqrt(deltaX * deltaX + deltaY * deltaY);
 
-    let zoomSign = Math.abs(deltaY) > Math.abs(deltaX) ?
+    const zoomSign = Math.abs(deltaY) > Math.abs(deltaX) ?
         Math.sign(deltaY) : Math.sign(-deltaX);
     if (zoomSign == 0) {
       return;
     }
 
-    let newScale = this.startScale_ * (1 + zoomSign * dist / 100);
-    let deltaCenterX = this.viewerBox_.width / 2 - centerClientX;
-    let deltaCenterY = this.viewerBox_.height / 2 - centerClientY;
+    const newScale = this.startScale_ * (1 + zoomSign * dist / 100);
+    const deltaCenterX = this.viewerBox_.width / 2 - centerClientX;
+    const deltaCenterY = this.viewerBox_.height / 2 - centerClientY;
     deltaX = Math.min(deltaCenterX, deltaCenterX * (dist / 100));
     deltaY = Math.min(deltaCenterY, deltaCenterY * (dist / 100));
     this.onZoom_(newScale, deltaX, deltaY, false);
@@ -504,15 +504,15 @@ export class ImageViewer {
    * @private
    */
   onZoom_(scale, deltaX, deltaY, animate) {
-    let newScale = this.boundScale_(scale, true);
+    const newScale = this.boundScale_(scale, true);
     if (newScale == this.scale_) {
       return;
     }
 
     this.updatePanZoomBounds_(newScale);
 
-    let newPosX = this.boundX_(this.startX_ + deltaX * newScale, false);
-    let newPosY = this.boundY_(this.startY_ + deltaY * newScale, false);
+    const newPosX = this.boundX_(this.startX_ + deltaX * newScale, false);
+    const newPosY = this.boundY_(this.startY_ + deltaY * newScale, false);
     return this.set_(newScale, newPosX, newPosY, animate);
   }
 
@@ -539,7 +539,7 @@ export class ImageViewer {
       }).thenAlways();
     }
 
-    let relayout = this.scale_ > this.startScale_;
+    const relayout = this.scale_ > this.startScale_;
     return promise.then(() => {
       return this.release_();
     }).then(() => {
@@ -559,14 +559,14 @@ export class ImageViewer {
    * @private
    */
   set_(newScale, newPosX, newPosY, animate) {
-    let ds = newScale - this.scale_;
-    let dx = newPosX - this.posX_;
-    let dy = newPosY - this.posY_;
-    let dist = Math.sqrt(dx * dx + dy * dy);
+    const ds = newScale - this.scale_;
+    const dx = newPosX - this.posX_;
+    const dy = newPosY - this.posY_;
+    const dist = Math.sqrt(dx * dx + dy * dy);
 
     let dur = 0;
     if (animate) {
-      let maxDur = 250;
+      const maxDur = 250;
       dur = Math.min(maxDur, Math.max(
           maxDur * dist * 0.01,      // Moving component.
           maxDur * Math.abs(ds)));   // Zooming component.
@@ -574,9 +574,9 @@ export class ImageViewer {
 
     let promise;
     if (dur > 16 && animate) {
-      let scaleFunc = tr.numeric(this.scale_, newScale);
-      let xFunc = tr.numeric(this.posX_, newPosX);
-      let yFunc = tr.numeric(this.posY_, newPosY);
+      const scaleFunc = tr.numeric(this.scale_, newScale);
+      const xFunc = tr.numeric(this.posX_, newPosX);
+      const yFunc = tr.numeric(this.posY_, newPosY);
       promise = Animation.animate(time => {
         this.scale_ = scaleFunc(time);
         this.posX_ = xFunc(time);
@@ -609,12 +609,12 @@ export class ImageViewer {
    * @private
    */
   release_() {
-    let newScale = this.boundScale_(this.scale_, false);
+    const newScale = this.boundScale_(this.scale_, false);
     if (newScale != this.scale_) {
       this.updatePanZoomBounds_(newScale);
     }
-    let newPosX = this.boundX_(this.posX_ / this.scale_ * newScale, false);
-    let newPosY = this.boundY_(this.posY_ / this.scale_ * newScale, false);
+    const newPosX = this.boundX_(this.posX_ / this.scale_ * newScale, false);
+    const newPosY = this.boundY_(this.posY_ / this.scale_ * newScale, false);
     return this.set_(newScale, newPosX, newPosY, true).then(() => {
       this.startScale_ = this.scale_;
       this.startX_ = this.posX_;
@@ -675,7 +675,7 @@ class AmpImageLightbox extends AMP.BaseElement {
     this.captionElement_.classList.add('-amp-image-lightbox-caption');
     this.container_.appendChild(this.captionElement_);
 
-    let gestures = Gestures.get(this.element);
+    const gestures = Gestures.get(this.element);
     this.element.addEventListener('click', e => {
       if (!this.entering_ &&
             !this.imageViewer_.getImage().contains(e.target)) {
@@ -698,7 +698,7 @@ class AmpImageLightbox extends AMP.BaseElement {
       return;
     }
 
-    let source = invocation.source;
+    const source = invocation.source;
     assert(source && SUPPORTED_ELEMENTS_[source.tagName.toLowerCase()],
         'Unsupported element: %s', source.tagName);
 
@@ -776,7 +776,7 @@ class AmpImageLightbox extends AMP.BaseElement {
 
     // 1. Check <figure> and <figcaption>.
     if (!caption) {
-      let figure = dom.closestByTag(sourceElement, 'figure');
+      const figure = dom.closestByTag(sourceElement, 'figure');
       if (figure) {
         caption = dom.elementByTag(figure, 'figcaption');
       }
@@ -784,7 +784,7 @@ class AmpImageLightbox extends AMP.BaseElement {
 
     // 2. Check "aria-describedby".
     if (!caption) {
-      let describedBy = sourceElement.getAttribute('aria-describedby');
+      const describedBy = sourceElement.getAttribute('aria-describedby');
       caption = document.getElementById(describedBy);
     }
 
@@ -816,8 +816,8 @@ class AmpImageLightbox extends AMP.BaseElement {
     });
     this.imageViewer_.measure();
 
-    let anim = new Animation();
-    let dur = 500;
+    const anim = new Animation();
+    const dur = 500;
 
     // Lightbox background fades in.
     anim.add(0, tr.setStyles(this.element, {
@@ -832,9 +832,9 @@ class AmpImageLightbox extends AMP.BaseElement {
       transLayer.classList.add('-amp-image-lightbox-trans');
       document.body.appendChild(transLayer);
 
-      let rect = layoutRectFromDomRect(this.sourceImage_
+      const rect = layoutRectFromDomRect(this.sourceImage_
           ./*OK*/getBoundingClientRect());
-      let clone = this.sourceImage_.cloneNode(true);
+      const clone = this.sourceImage_.cloneNode(true);
       st.setStyles(clone, {
         position: 'absolute',
         top: st.px(rect.top),
@@ -847,12 +847,12 @@ class AmpImageLightbox extends AMP.BaseElement {
       this.sourceImage_.classList.add('-amp-ghost');
 
       // Move the image to the location given by the lightbox.
-      let imageBox = this.imageViewer_.getImageBox();
-      let dx = imageBox.left - rect.left;
-      let dy = imageBox.top - rect.top;
+      const imageBox = this.imageViewer_.getImageBox();
+      const dx = imageBox.left - rect.left;
+      const dy = imageBox.top - rect.top;
       // Duration will be somewhere between 0.2 and 0.8 depending on how far
       // the image needs to move.
-      let motionTime = Math.max(0.2, Math.min(0.8, Math.abs(dy) / 250 * 0.8));
+      const motionTime = Math.max(0.2, Math.min(0.8, Math.abs(dy) / 250 * 0.8));
       anim.add(0, tr.setStyles(clone, {
         transform: tr.translate(tr.numeric(0, dx), tr.numeric(0, dy))
       }), motionTime, ENTER_CURVE_);
@@ -884,10 +884,10 @@ class AmpImageLightbox extends AMP.BaseElement {
    * @private
    */
   exit_() {
-    let image = this.imageViewer_.getImage();
-    let imageBox = this.imageViewer_.getImageBoxWithOffset();
+    const image = this.imageViewer_.getImage();
+    const imageBox = this.imageViewer_.getImageBoxWithOffset();
 
-    let anim = new Animation();
+    const anim = new Animation();
     let dur = 500;
 
     // Lightbox background fades out.
@@ -902,11 +902,11 @@ class AmpImageLightbox extends AMP.BaseElement {
       transLayer.classList.add('-amp-image-lightbox-trans');
       document.body.appendChild(transLayer);
 
-      let rect = layoutRectFromDomRect(this.sourceImage_
+      const rect = layoutRectFromDomRect(this.sourceImage_
           ./*OK*/getBoundingClientRect());
-      let newLeft = imageBox.left + (imageBox.width - rect.width) / 2;
-      let newTop = imageBox.top + (imageBox.height - rect.height) / 2;
-      let clone = image.cloneNode(true);
+      const newLeft = imageBox.left + (imageBox.width - rect.width) / 2;
+      const newTop = imageBox.top + (imageBox.height - rect.height) / 2;
+      const clone = image.cloneNode(true);
       st.setStyles(clone, {
         position: 'absolute',
         top: st.px(newTop),
@@ -923,15 +923,15 @@ class AmpImageLightbox extends AMP.BaseElement {
       }), 0.1, EXIT_CURVE_);
 
       // Move the image back to where it is in the article.
-      let dx = rect.left - newLeft;
-      let dy = rect.top - newTop;
-      let move = tr.setStyles(clone, {
+      const dx = rect.left - newLeft;
+      const dy = rect.top - newTop;
+      const move = tr.setStyles(clone, {
         transform: tr.translate(tr.numeric(0, dx), tr.numeric(0, dy))
       });
       // Duration will be somewhere between 0.2 and 0.8 depending on how far
       // the image needs to move. Start the motion later too, but no later
       // than 0.2.
-      let motionTime = Math.max(0.2, Math.min(0.8, Math.abs(dy) / 250 * 0.8));
+      const motionTime = Math.max(0.2, Math.min(0.8, Math.abs(dy) / 250 * 0.8));
       anim.add(Math.min(0.8 - motionTime, 0.2), (time, complete) => {
         move(time);
         if (complete) {
diff --git a/extensions/amp-image-lightbox/0.1/test/test-amp-image-lightbox.js b/extensions/amp-image-lightbox/0.1/test/test-amp-image-lightbox.js
index 4f0f9c6a1276..a848cb1ea33e 100644
--- a/extensions/amp-image-lightbox/0.1/test/test-amp-image-lightbox.js
+++ b/extensions/amp-image-lightbox/0.1/test/test-amp-image-lightbox.js
@@ -42,22 +42,23 @@ describe('amp-image-lightbox component', () => {
 
   it('should render correctly', () => {
     return getImageLightbox().then(lightbox => {
-      let container = lightbox.querySelector('.-amp-image-lightbox-container');
+      const container = lightbox
+          .querySelector('.-amp-image-lightbox-container');
       expect(container).to.not.equal(null);
 
-      let caption = container.querySelector('.-amp-image-lightbox-caption');
+      const caption = container.querySelector('.-amp-image-lightbox-caption');
       expect(caption).to.not.equal(null);
       expect(caption.classList.contains('amp-image-lightbox-caption')).to
           .equal(true);
 
-      let viewer = container.querySelector('.-amp-image-lightbox-viewer');
+      const viewer = container.querySelector('.-amp-image-lightbox-viewer');
       expect(viewer).to.not.equal(null);
 
-      let image = viewer.querySelector('.-amp-image-lightbox-viewer-image');
+      const image = viewer.querySelector('.-amp-image-lightbox-viewer-image');
       expect(image).to.not.equal(null);
 
       // Very important. Image must have transform-origin=50% 50%.
-      let win = image.ownerDocument.defaultView;
+      const win = image.ownerDocument.defaultView;
       expect(win.getComputedStyle(image)['transform-origin']).to.equal(
           '50% 50%');
     });
@@ -65,26 +66,26 @@ describe('amp-image-lightbox component', () => {
 
   it('should activate all steps', () => {
     return getImageLightbox().then(lightbox => {
-      let impl = lightbox.implementation_;
-      let requestFullOverlay = sinon.spy();
+      const impl = lightbox.implementation_;
+      const requestFullOverlay = sinon.spy();
       impl.requestFullOverlay = requestFullOverlay;
-      let viewportOnChanged = sinon.spy();
-      let disableTouchZoom = sinon.spy();
+      const viewportOnChanged = sinon.spy();
+      const disableTouchZoom = sinon.spy();
       impl.getViewport = () => {return {
         onChanged: viewportOnChanged,
         disableTouchZoom: disableTouchZoom
       };};
-      let historyPush = sinon.spy();
+      const historyPush = sinon.spy();
       impl.getHistory_ = () => {
         return {push: () => {
           historyPush();
           return Promise.resolve(11);
         }};
       };
-      let enter = sinon.spy();
+      const enter = sinon.spy();
       impl.enter_ = enter;
 
-      let ampImage = document.createElement('amp-img');
+      const ampImage = document.createElement('amp-img');
       ampImage.setAttribute('src', 'data:');
       impl.activate({source: ampImage});
 
@@ -100,25 +101,25 @@ describe('amp-image-lightbox component', () => {
 
   it('should deactivate all steps', () => {
     return getImageLightbox().then(lightbox => {
-      let impl = lightbox.implementation_;
+      const impl = lightbox.implementation_;
       impl.active_ = true;
       impl.historyId_ = 11;
-      let cancelFullOverlay = sinon.spy();
+      const cancelFullOverlay = sinon.spy();
       impl.cancelFullOverlay = cancelFullOverlay;
-      let viewportOnChangedUnsubscribed = sinon.spy();
+      const viewportOnChangedUnsubscribed = sinon.spy();
       impl.unlistenViewport_ = viewportOnChangedUnsubscribed;
-      let restoreOriginalTouchZoom = sinon.spy();
+      const restoreOriginalTouchZoom = sinon.spy();
       impl.getViewport = () => {return {
         restoreOriginalTouchZoom: restoreOriginalTouchZoom
       };};
-      let historyPop = sinon.spy();
+      const historyPop = sinon.spy();
       impl.getHistory_ = () => {
         return {pop: historyPop};
       };
-      let exit = sinon.spy();
+      const exit = sinon.spy();
       impl.exit_ = exit;
 
-      let ampImage = document.createElement('amp-img');
+      const ampImage = document.createElement('amp-img');
       ampImage.setAttribute('src', 'data:');
       impl.close();
 
@@ -177,7 +178,7 @@ describe('amp-image-lightbox image viewer', () => {
 
 
   it('should init to the source element without image', () => {
-    let sourceElement = {
+    const sourceElement = {
       offsetWidth: 101,
       offsetHeight: 201,
       getAttribute: name => {
@@ -197,7 +198,7 @@ describe('amp-image-lightbox image viewer', () => {
   });
 
   it('should init to the source element with unloaded image', () => {
-    let sourceElement = {
+    const sourceElement = {
       offsetWidth: 101,
       offsetHeight: 201,
       getAttribute: name => {
@@ -207,7 +208,7 @@ describe('amp-image-lightbox image viewer', () => {
         return undefined;
       }
     };
-    let sourceImage = {
+    const sourceImage = {
       complete: false,
       src: 'image1-smaller'
     };
@@ -218,7 +219,7 @@ describe('amp-image-lightbox image viewer', () => {
   });
 
   it('should init to the source element with loaded image', () => {
-    let sourceElement = {
+    const sourceElement = {
       offsetWidth: 101,
       offsetHeight: 201,
       getAttribute: name => {
@@ -228,7 +229,7 @@ describe('amp-image-lightbox image viewer', () => {
         return undefined;
       }
     };
-    let sourceImage = {
+    const sourceImage = {
       complete: true,
       src: 'image1-smaller'
     };
@@ -261,7 +262,7 @@ describe('amp-image-lightbox image viewer', () => {
     imageViewer.sourceWidth_ = 80;
     imageViewer.sourceHeight_ = 60;
 
-    let promise = imageViewer.measure();
+    const promise = imageViewer.measure();
 
     expect(imageViewer.viewerBox_.width).to.equal(100);
     expect(imageViewer.viewerBox_.height).to.equal(200);
@@ -277,7 +278,7 @@ describe('amp-image-lightbox image viewer', () => {
     expect(imageViewer.getImage().style.height).to.equal('75px');
 
     clock.tick(10);
-    let checkSrc = () => {
+    const checkSrc = () => {
       expect(imageViewer.getImage().getAttribute('src')).to.equal('image1');
     };
     return promise.then(checkSrc, checkSrc);
@@ -422,7 +423,7 @@ describe('amp-image-lightbox image viewer gestures', () => {
   });
 
   it('should zoom release', () => {
-    let updateSrc = sinon.spy();
+    const updateSrc = sinon.spy();
     imageViewer.updateSrc_ = updateSrc;
     imageViewer.onZoomInc_(10, 10, -10, -10);
     return imageViewer.onZoomRelease_(10, 10, -10, -10, 0, 0).then(() => {
diff --git a/extensions/amp-lightbox/0.1/amp-lightbox.js b/extensions/amp-lightbox/0.1/amp-lightbox.js
index 1cda6965b1ba..76508ba4ca83 100644
--- a/extensions/amp-lightbox/0.1/amp-lightbox.js
+++ b/extensions/amp-lightbox/0.1/amp-lightbox.js
@@ -46,7 +46,7 @@ class AmpLightbox extends AMP.BaseElement {
       right: 0
     });
 
-    let children = this.getRealChildren();
+    const children = this.getRealChildren();
 
     /** @private {!Element} */
     this.container_ = document.createElement('div');
@@ -56,7 +56,7 @@ class AmpLightbox extends AMP.BaseElement {
       this.container_.appendChild(child);
     });
 
-    let gestures = Gestures.get(this.element);
+    const gestures = Gestures.get(this.element);
     // TODO(dvoytenko): configure how to close. Or maybe leave it completely
     // up to "on" element.
     this.element.addEventListener('click', () => this.close());
diff --git a/extensions/amp-pinterest/0.1/amp-pinterest.js b/extensions/amp-pinterest/0.1/amp-pinterest.js
index a036bccbfcb3..fda021b6f59d 100644
--- a/extensions/amp-pinterest/0.1/amp-pinterest.js
+++ b/extensions/amp-pinterest/0.1/amp-pinterest.js
@@ -60,15 +60,15 @@ class AmpPinterest extends AMP.BaseElement {
   /** @override */
   layoutCallback() {
 
-    let height = this.element.getAttribute('height');
+    const height = this.element.getAttribute('height');
 
-    let width = this.element.getAttribute('width');
+    const width = this.element.getAttribute('width');
 
-    let pinDo = AMP.assert(this.element.getAttribute('data-do'),
+    const pinDo = AMP.assert(this.element.getAttribute('data-do'),
       'The data-do attribute is required for <amp-pinterest> %s',
       this.element);
 
-    let pinUrl = AMP.assert(this.element.getAttribute('data-url'),
+    const pinUrl = AMP.assert(this.element.getAttribute('data-url'),
       'The data-url attribute is required for <amp-pinterest> %s',
       this.element);
 
@@ -79,7 +79,7 @@ class AmpPinterest extends AMP.BaseElement {
     }
 
     // open a new window
-    let pop = function(url, shouldPop) {
+    const pop = function(url, shouldPop) {
       if (shouldPop) {
         // amp=1&guid=guid are already in the query before long fields
         window.open(url, '_pinit', POP);
@@ -90,8 +90,8 @@ class AmpPinterest extends AMP.BaseElement {
     };
 
     // log a string
-    let log = function(str) {
-      let call = new Image();
+    const log = function(str) {
+      const call = new Image();
       let query = 'https://log.pinterest.com/?guid=' + guid;
       query = query + '&amp=1';
       // add optional string &foo=bar
@@ -103,15 +103,15 @@ class AmpPinterest extends AMP.BaseElement {
     };
 
     // load data from API via XHR
-    let call = function(url) {
+    const call = function(url) {
       return new Promise((resolve, reject) => {
-        let xhr = new XMLHttpRequest();
+        const xhr = new XMLHttpRequest();
         xhr.onreadystatechange = function() {
           if (xhr.readyState == 4) {
             if (xhr.status == 200) {
               // all replies are JSON and should parse
               try {
-                let data = JSON.parse(xhr.responseText);
+                const data = JSON.parse(xhr.responseText);
                 resolve(data);
               } catch (err) {
                 reject(new Error('API reply was not JSON: ' + url));
@@ -127,7 +127,7 @@ class AmpPinterest extends AMP.BaseElement {
     };
 
     // set a DOM property or text attribute
-    let set = function(el, att, string) {
+    const set = function(el, att, string) {
       if (typeof el[att] === 'string') {
         el[att] = string;
       } else {
@@ -136,7 +136,7 @@ class AmpPinterest extends AMP.BaseElement {
     };
 
     // create a DOM element
-    let make = function(obj) {
+    const make = function(obj) {
       let el = false, tag, att;
       for (tag in obj) {
         el = document.createElement(tag);
@@ -151,7 +151,7 @@ class AmpPinterest extends AMP.BaseElement {
     };
 
     // filter tags from API input
-    let filter = function(str) {
+    const filter = function(str) {
       let decoded, ret;
       decoded = '';
       ret = '';
@@ -164,9 +164,9 @@ class AmpPinterest extends AMP.BaseElement {
     };
 
     // save our outside context so we can return properly after rendering
-    let that = this;
+    const that = this;
 
-    let makePin = function() {
+    const makePin = function() {
 
       // make an embedded pin widget
       // pinId will be inferred from pinUrl
@@ -179,14 +179,14 @@ class AmpPinterest extends AMP.BaseElement {
         return;
       }
 
-      let width = that.element.getAttribute('data-width');
+      const width = that.element.getAttribute('data-width');
 
-      let structure = make({'span': {}});
+      const structure = make({'span': {}});
 
-      let renderPin = function(r) {
+      const renderPin = function(r) {
 
         if (r && r.data && r.data[0] && !r.data[0].error) {
-          let p = r.data[0];
+          const p = r.data[0];
 
           // start setting our class name
           let className = '-amp-pinterest-embed-pin';
@@ -203,12 +203,12 @@ class AmpPinterest extends AMP.BaseElement {
 
           structure.className = className;
 
-          let container = make({'span': {
+          const container = make({'span': {
             'className': '-amp-pinterest-embed-pin-inner',
             'data-pin-log': 'embed_pin'
           }});
 
-          let img = make({'img': {
+          const img = make({'img': {
             'src': imgUrl,
             'className': '-amp-pinterest-embed-pin-image',
             'data-pin-no-hover': true,
@@ -218,7 +218,7 @@ class AmpPinterest extends AMP.BaseElement {
           container.appendChild(img);
 
           // repin button
-          let repin = make({'span': {
+          const repin = make({'span': {
             'className': '-amp-pinterest-rect -amp-pinterest-en-red' +
               ' -amp-pinterest-embed-pin-repin',
             'data-pin-log': 'embed_pin_repin',
@@ -229,13 +229,13 @@ class AmpPinterest extends AMP.BaseElement {
           container.appendChild(repin);
 
           // text container
-          let text = make({'span': {
+          const text = make({'span': {
             'className': '-amp-pinterest-embed-pin-text'
           }});
 
           // description
           if (p.description) {
-            let description = make({'span': {
+            const description = make({'span': {
               'className': '-amp-pinterest-embed-pin-text-block ' +
                 '-amp-pinterest-embed-pin-description',
               'textContent': filter(p.description)
@@ -245,7 +245,7 @@ class AmpPinterest extends AMP.BaseElement {
 
           // attribution
           if (p.attribution) {
-            let attribution = make({'span': {
+            const attribution = make({'span': {
               'className': '-amp-pinterest-embed-pin-text-block' +
                 ' -amp-pinterest-embed-pin-attribution'
             }});
@@ -265,12 +265,12 @@ class AmpPinterest extends AMP.BaseElement {
 
           // likes and repins
           if (p.repin_count || p.like_count) {
-            let stats = make({'span': {
+            const stats = make({'span': {
               'className': '-amp-pinterest-embed-pin-text-block' +
                 ' -amp-pinterest-embed-pin-stats'
             }});
             if (p.repin_count) {
-              let repinCount = make({'span': {
+              const repinCount = make({'span': {
                 'className': '-amp-pinterest-embed-pin-stats-repins',
                 'textContent': String(p.repin_count)
               }});
@@ -278,7 +278,7 @@ class AmpPinterest extends AMP.BaseElement {
             }
 
             if (p.like_count) {
-              let likeCount = make({'span': {
+              const likeCount = make({'span': {
                 'className': '-amp-pinterest-embed-pin-stats-likes',
                 'textContent': String(p.like_count)
               }});
@@ -290,7 +290,7 @@ class AmpPinterest extends AMP.BaseElement {
           // pinner
           if (p.pinner) {
 
-            let pinner = make({'span': {
+            const pinner = make({'span': {
               'className': '-amp-pinterest-embed-pin-text-block' +
                 ' -amp-pinterest-embed-pin-pinner'
             }});
@@ -327,14 +327,14 @@ class AmpPinterest extends AMP.BaseElement {
 
           // listen for clicks
           structure.addEventListener('click', function(e) {
-            let el = e.target;
-            let logMe = el.getAttribute('data-pin-log');
+            const el = e.target;
+            const logMe = el.getAttribute('data-pin-log');
             if (logMe) {
               log('&type=' + logMe);
             }
-            let href = el.getAttribute('data-pin-href');
+            const href = el.getAttribute('data-pin-href');
             if (href) {
-              let popThis = el.getAttribute('data-pin-pop') || false;
+              const popThis = el.getAttribute('data-pin-pop') || false;
               pop(href, popThis);
             }
             e.preventDefault();
@@ -349,7 +349,7 @@ class AmpPinterest extends AMP.BaseElement {
         }
       };
 
-      let query = 'https://widgets.pinterest.com/v3/pidgets/pins/info/' +
+      const query = 'https://widgets.pinterest.com/v3/pidgets/pins/info/' +
         '?pin_ids=' + pinId +
         '&sub=www&base_scheme=https';
       return call(query).then(r => {
@@ -357,31 +357,31 @@ class AmpPinterest extends AMP.BaseElement {
       });
     };
 
-    let makeButton = function() {
+    const makeButton = function() {
 
       // render a Pin It button
       // required: media and description
       // optional: round, color, tall, lang, count
 
-      let pinMedia =
+      const pinMedia =
         AMP.assert(that.element.getAttribute('data-media'),
         'The data-media attribute is required when <amp-pinterest> ' +
         'makes a Pin It button %s', that.element);
 
-      let pinDescription =
+      const pinDescription =
          AMP.assert(that.element.getAttribute('data-description'),
         'The data-description attribute is required when <amp-pinterest> ' +
         'makes a Pin It button %s', that.element);
 
       // options
-      let round = that.element.getAttribute('data-round');
+      const round = that.element.getAttribute('data-round');
       let color = that.element.getAttribute('data-color');
-      let tall = that.element.getAttribute('data-tall');
+      const tall = that.element.getAttribute('data-tall');
       let lang = that.element.getAttribute('data-lang');
-      let count = that.element.getAttribute('data-count');
+      const count = that.element.getAttribute('data-count');
 
       // pass a known guid when testing
-      let theGuid = that.element.getAttribute('data-volkswagen-guid') || guid;
+      const theGuid = that.element.getAttribute('data-volkswagen-guid') || guid;
 
       // build our link
       let link = 'https://www.pinterest.com/pin/create/button/';
@@ -392,15 +392,15 @@ class AmpPinterest extends AMP.BaseElement {
         encodeURIComponent(pinDescription);
 
       // start building a link
-      let a = make({'A': {
+      const a = make({'A': {
         'href': link
       }});
 
       // built it
-      let render = function(r) {
+      const render = function(r) {
 
         // shorten the pin count so it will fit in our bubble
-        let prettyPinCount = function(n) {
+        const prettyPinCount = function(n) {
           if (n > 999) {
             if (n < 1000000) {
               n = parseInt(n / 1000, 10) + 'K+';
@@ -460,7 +460,7 @@ class AmpPinterest extends AMP.BaseElement {
 
         if (r) {
 
-          let countBubble = document.createElement('SPAN');
+          const countBubble = document.createElement('SPAN');
 
           // position the count
           if (count === 'above') {
@@ -511,7 +511,7 @@ class AmpPinterest extends AMP.BaseElement {
       // only ask for a count if the operator requires it
       if (count === 'above' || count === 'beside') {
         // call the API
-        let query = 'https://widgets.pinterest.com/v1/urls/count.json?' +
+        const query = 'https://widgets.pinterest.com/v1/urls/count.json?' +
           '&return_jsonp=false' +
           '&url=' + encodeURIComponent(pinUrl);
         return call(query).then(r => {
diff --git a/src/action.js b/src/action.js
index 1c7f4b14e32e..61ef1e29e729 100644
--- a/src/action.js
+++ b/src/action.js
@@ -128,13 +128,13 @@ export class Action {
    * @private
    */
   action_(source, actionEventType, event) {
-    let action = this.findAction_(source, actionEventType);
+    const action = this.findAction_(source, actionEventType);
     if (!action) {
       // TODO(dvoytenko): implement default (catch-all) actions.
       return;
     }
 
-    let target = document.getElementById(action.actionInfo.target);
+    const target = document.getElementById(action.actionInfo.target);
     if (!target) {
       this.actionInfoError_('target not found', action.actionInfo, target);
       return;
@@ -166,7 +166,7 @@ export class Action {
    * @param {?ActionInfo} actionInfo
    */
   invoke_(target, method, source, event, actionInfo) {
-    let invocation = new ActionInvocation(target, method, source, event);
+    const invocation = new ActionInvocation(target, method, source, event);
 
     // TODO(dvoytenko): implement common method handlers, e.g. "toggleClass"
 
@@ -213,7 +213,7 @@ export class Action {
    * @return {?ActionInfo_}
    */
   matchActionInfo_(node, actionEventType) {
-    let actionMap = this.getActionMap_(node);
+    const actionMap = this.getActionMap_(node);
     if (!actionMap) {
       return null;
     }
@@ -242,11 +242,11 @@ export class Action {
    */
   parseActionMap_(s) {
     let actionMap = null;
-    let actions = s.split(';');
+    const actions = s.split(';');
     if (actions && actions.length > 0) {
       for (let i = 0; i < actions.length; i++) {
-        let actionStr = actions[i];
-        let actionInfo = this.parseAction_(actionStr);
+        const actionStr = actions[i];
+        const actionInfo = this.parseAction_(actionStr);
         if (actionInfo) {
           if (!actionMap) {
             actionMap = {};
@@ -268,13 +268,13 @@ export class Action {
       return null;
     }
 
-    let eventSep = s.indexOf(':');
-    let methodSep = s.indexOf('.', eventSep + 1);
-    let event = (eventSep != -1 ? s.substring(0, eventSep) : '').toLowerCase()
+    const eventSep = s.indexOf(':');
+    const methodSep = s.indexOf('.', eventSep + 1);
+    const event = (eventSep != -1 ? s.substring(0, eventSep) : '').toLowerCase()
         .trim() || null;
-    let target = s.substring(eventSep + 1, methodSep != -1 ? methodSep :
+    const target = s.substring(eventSep + 1, methodSep != -1 ? methodSep :
         s.length).trim();
-    let method = (methodSep != -1 ? s.substring(methodSep + 1) : '')
+    const method = (methodSep != -1 ? s.substring(methodSep + 1) : '')
         .trim() || DEFAULT_METHOD_;
 
     if (!event || !target) {
diff --git a/src/animation.js b/src/animation.js
index 005701ec6089..81cad4e89efe 100644
--- a/src/animation.js
+++ b/src/animation.js
@@ -19,9 +19,9 @@ import {log} from './log';
 import {timer} from './timer';
 import {vsync} from './vsync';
 
-let TAG_ = 'Animation';
+const TAG_ = 'Animation';
 
-let NOOP_CALLBACK = function() {};
+const NOOP_CALLBACK = function() {};
 
 
 /**
@@ -106,7 +106,7 @@ export class Animation {
    * @return {!AnimationPlayer}
    */
   start(duration) {
-    let player = new AnimationPlayer(this.vsync_, this.segments_, this.curve_,
+    const player = new AnimationPlayer(this.vsync_, this.segments_, this.curve_,
         duration);
     player.start_();
     return player;
@@ -135,7 +135,7 @@ class AnimationPlayer {
     /** @private @const {!Array<!SegmentRuntime_>} */
     this.segments_ = [];
     for (let i = 0; i < segments.length; i++) {
-      let segment = segments[i];
+      const segment = segments[i];
       this.segments_.push({
         delay: segment.delay,
         func: segment.func,
@@ -205,7 +205,7 @@ class AnimationPlayer {
    * @return {!Promise}
    */
   thenAlways(opt_callback) {
-    let callback = opt_callback || NOOP_CALLBACK;
+    const callback = opt_callback || NOOP_CALLBACK;
     return this.then(callback, callback);
   }
 
@@ -281,13 +281,13 @@ class AnimationPlayer {
     if (!this.running_) {
       return;
     }
-    let currentTime = timer.now();
-    let normLinearTime = Math.min((currentTime - this.startTime_) /
+    const currentTime = timer.now();
+    const normLinearTime = Math.min((currentTime - this.startTime_) /
         this.duration_, 1);
 
     // Start segments due to be started
     for (let i = 0; i < this.segments_.length; i++) {
-      let segment = this.segments_[i];
+      const segment = this.segments_[i];
       if (!segment.started && normLinearTime >= segment.delay) {
         segment.started = true;
       }
@@ -295,7 +295,7 @@ class AnimationPlayer {
 
     // Execute all pending segments.
     for (let i = 0; i < this.segments_.length; i++) {
-      let segment = this.segments_[i];
+      const segment = this.segments_[i];
       if (!segment.started || segment.completed) {
         continue;
       }
diff --git a/src/base-element.js b/src/base-element.js
index e9c2df3bde04..3bb7c446e112 100644
--- a/src/base-element.js
+++ b/src/base-element.js
@@ -320,7 +320,7 @@ export class BaseElement {
     if (invocation.method == 'activate') {
       this.activate(invocation);
     } else {
-      let handler = this.actionMap_[invocation.method];
+      const handler = this.actionMap_[invocation.method];
       if (!handler) {
         throw new Error(`Method not found: ${invocation.method}`);
       }
@@ -353,7 +353,7 @@ export class BaseElement {
    */
   propagateAttributes(attributes, element) {
     for (let i = 0; i < attributes.length; i++) {
-      let attr = attributes[i];
+      const attr = attributes[i];
       if (!this.element.hasAttribute(attr)) {
         continue;
       }
diff --git a/src/curve.js b/src/curve.js
index efca09c2285b..50bdc08d9f99 100644
--- a/src/curve.js
+++ b/src/curve.js
@@ -32,7 +32,7 @@ class Curve {};
  * @return {!Curve}
  */
 export function bezierCurve(x1, y1, x2, y2) {
-  let bezier = new Bezier(0, 0, x1, y1, x2, y2, 1, 1);
+  const bezier = new Bezier(0, 0, x1, y1, x2, y2, 1, 1);
   return bezier.solveYValueFromXValue.bind(bezier);
 }
 
diff --git a/src/custom-element.js b/src/custom-element.js
index b0b3254a3981..e0267ca16579 100644
--- a/src/custom-element.js
+++ b/src/custom-element.js
@@ -55,7 +55,7 @@ const PREPARE_LOADING_THRESHOLD_ = 1000;
  * Map from element name to implementation class.
  * @const {Object}
  */
-let knownElements = {};
+const knownElements = {};
 
 
 /**
@@ -72,7 +72,7 @@ export function upgradeOrRegisterElement(win, name, toClass) {
   assert(knownElements[name] == ElementStub,
       'Expected ' + name + ' to be an ElementStub.');
   for (let i = 0; i < stubbedElements.length; i++) {
-    let stub = stubbedElements[i];
+    const stub = stubbedElements[i];
     // There are 3 possible states here:
     // 1. We never made the stub because the extended impl. loaded first.
     //    In that case the element won't be in the array.
@@ -98,9 +98,9 @@ export function upgradeOrRegisterElement(win, name, toClass) {
  * @param {!Window} win
  */
 export function stubElements(win) {
-  let list = win.document.querySelectorAll('[custom-element]');
+  const list = win.document.querySelectorAll('[custom-element]');
   for (let i = 0; i < list.length; i++) {
-    let name = list[i].getAttribute('custom-element');
+    const name = list[i].getAttribute('custom-element');
     if (knownElements[name]) {
       continue;
     }
@@ -116,15 +116,15 @@ export function stubElements(win) {
 export function applyLayout_(element) {
   let widthAttr = element.getAttribute('width');
   let heightAttr = element.getAttribute('height');
-  let sizesAttr = element.getAttribute('sizes');
-  let layoutAttr = element.getAttribute('layout');
+  const sizesAttr = element.getAttribute('sizes');
+  const layoutAttr = element.getAttribute('layout');
 
   // Handle elements that do not specify a width/height and are defined to have
   // natural browser dimensions.
   if ((!layoutAttr || layoutAttr == Layout.FIXED ||
           layoutAttr == Layout.FIXED_HEIGHT) &&
       (!widthAttr || !heightAttr) && hasNaturalDimensions(element.tagName)) {
-    let dimensions = getNaturalDimensions(element.tagName);
+    const dimensions = getNaturalDimensions(element.tagName);
     if (layoutAttr != Layout.FIXED_HEIGHT) {
       widthAttr = widthAttr || dimensions.width;
     }
@@ -168,7 +168,7 @@ export function applyLayout_(element) {
             'integer/length value: ' + widthAttr);
       }
     }
-    let height = parseLength(heightAttr);
+    const height = parseLength(heightAttr);
     if (!height) {
       throw new Error('Expected height to be available and be an ' +
           'integer/length value: ' + heightAttr);
@@ -178,7 +178,7 @@ export function applyLayout_(element) {
         throw new Error('Length units should be the same for width ' + width +
             ' and height ' + height);
       }
-      let sizer = element.ownerDocument.createElement('i-amp-sizer');
+      const sizer = element.ownerDocument.createElement('i-amp-sizer');
       sizer.style.display = 'block';
       sizer.style.paddingTop =
           ((getLengthNumeral(height) / getLengthNumeral(width)) * 100) + '%';
@@ -332,8 +332,8 @@ export function createAmpElementProto(win, name, implementationClass) {
    * @final @package
    */
   ElementProto.upgrade = function(newImplClass) {
-    let registeredStub = this.implementation_;
-    let newImpl = new newImplClass(this);
+    const registeredStub = this.implementation_;
+    const newImpl = new newImplClass(this);
     this.implementation_ = newImpl;
     if (registeredStub) {
       registeredStub.upgrade(newImpl);
@@ -465,7 +465,7 @@ export function createAmpElementProto(win, name, implementationClass) {
 
     // Sizes.
     if (this.sizeList_ === undefined) {
-      let sizesAttr = this.getAttribute('sizes');
+      const sizesAttr = this.getAttribute('sizes');
       this.sizeList_ = sizesAttr ? parseSizeList(sizesAttr) : null;
     }
     if (this.sizeList_) {
@@ -714,7 +714,7 @@ export function createAmpElementProto(win, name, implementationClass) {
       return;
     }
 
-    let actionQueue = assert(this.actionQueue_);
+    const actionQueue = assert(this.actionQueue_);
     this.actionQueue_ = null;
 
     actionQueue.forEach(invocation => {
@@ -746,7 +746,7 @@ export function createAmpElementProto(win, name, implementationClass) {
    * @package @final
    */
   ElementProto.getRealChildNodes = function() {
-    let nodes = [];
+    const nodes = [];
     for (let n = this.firstChild; n; n = n.nextSibling) {
       if (!isInternalOrServiceNode(n)) {
         nodes.push(n);
@@ -762,9 +762,9 @@ export function createAmpElementProto(win, name, implementationClass) {
    * @package @final
    */
   ElementProto.getRealChildren = function() {
-    let elements = [];
+    const elements = [];
     for (let i = 0; i < this.children.length; i++) {
-      let child = this.children[i];
+      const child = this.children[i];
       if (!isInternalOrServiceNode(child)) {
         elements.push(child);
       }
@@ -787,7 +787,7 @@ export function createAmpElementProto(win, name, implementationClass) {
    * @package @final
    */
   ElementProto.togglePlaceholder = function(state) {
-    let placeholder = this.getPlaceholder();
+    const placeholder = this.getPlaceholder();
     if (placeholder) {
       placeholder.classList.toggle('amp-hidden', !state);
     }
@@ -842,12 +842,12 @@ export function createAmpElementProto(win, name, implementationClass) {
    */
   ElementProto.prepareLoading_ = function() {
     if (!this.loadingContainer_) {
-      let container = document.createElement('div');
+      const container = document.createElement('div');
       container.classList.add('-amp-loading-container');
       container.classList.add('-amp-fill-content');
       container.classList.add('amp-hidden');
 
-      let element = createLoaderElement();
+      const element = createLoaderElement();
       container.appendChild(element);
 
       this.appendChild(container);
@@ -884,7 +884,7 @@ export function createAmpElementProto(win, name, implementationClass) {
       this.loadingElement_.classList.toggle('amp-active', state);
 
       if (!state && opt_cleanup) {
-        let loadingContainer = this.loadingContainer_;
+        const loadingContainer = this.loadingContainer_;
         this.loadingContainer_ = null;
         this.loadingElement_ = null;
         this.resources_.deferMutate(this, () => {
diff --git a/src/document-click.js b/src/document-click.js
index 1434714c4579..232139ee0ab4 100644
--- a/src/document-click.js
+++ b/src/document-click.js
@@ -97,23 +97,23 @@ export function onDocumentElementClick_(e, viewport) {
     return;
   }
 
-  let target = closestByTag(e.target, 'A');
+  const target = closestByTag(e.target, 'A');
   if (!target) {
     return;
   }
 
   let elem = null;
-  let docElement = e.currentTarget;
-  let doc = docElement.ownerDocument;
+  const docElement = e.currentTarget;
+  const doc = docElement.ownerDocument;
 
-  let tgtLoc = parseUrl(target.href);
+  const tgtLoc = parseUrl(target.href);
   if (!tgtLoc.hash) {
     return;
   }
 
-  let curLoc = parseUrl(doc.location.href);
-  let tgtHref = `${tgtLoc.origin}${tgtLoc.pathname}${tgtLoc.search}`;
-  let curHref = `${curLoc.origin}${curLoc.pathname}${curLoc.search}`;
+  const curLoc = parseUrl(doc.location.href);
+  const tgtHref = `${tgtLoc.origin}${tgtLoc.pathname}${tgtLoc.search}`;
+  const curHref = `${curLoc.origin}${curLoc.pathname}${curLoc.search}`;
 
   // If the current target anchor link is the same origin + path
   // as the current document then we know we are just linking to an
@@ -127,7 +127,7 @@ export function onDocumentElementClick_(e, viewport) {
   // history which contains the amp document.
   e.preventDefault();
 
-  let hash = tgtLoc.hash.slice(1);
+  const hash = tgtLoc.hash.slice(1);
   elem = doc.getElementById(hash);
 
   if (!elem) {
diff --git a/src/document-state.js b/src/document-state.js
index 3cb4af9f050c..b94ce467d5e9 100644
--- a/src/document-state.js
+++ b/src/document-state.js
@@ -86,7 +86,7 @@ export class DocumentState {
     this.visibilityChangeEvent_ = null;
     if (this.hiddenProp_) {
       this.visibilityChangeEvent_ = 'visibilitychange';
-      let vendorStop = this.hiddenProp_.indexOf('Hidden');
+      const vendorStop = this.hiddenProp_.indexOf('Hidden');
       if (vendorStop != -1) {
         this.visibilityChangeEvent_ =
             this.hiddenProp_.substring(0, vendorStop) + 'Visibilitychange';
diff --git a/src/dom.js b/src/dom.js
index 014f49828db3..b12afa2364d9 100644
--- a/src/dom.js
+++ b/src/dom.js
@@ -45,7 +45,7 @@ export function removeChildren(parent) {
  * @param {!Element} to
  */
 export function copyChildren(from, to) {
-  let frag = to.ownerDocument.createDocumentFragment();
+  const frag = to.ownerDocument.createDocumentFragment();
   for (let n = from.firstChild; n; n = n.nextSibling) {
     frag.appendChild(n.cloneNode(true));
   }
@@ -92,7 +92,7 @@ export function closestByTag(element, tagName) {
  * @return {?Element}
  */
 export function elementByTag(element, tagName) {
-  let elements = element.getElementsByTagName(tagName);
+  const elements = element.getElementsByTagName(tagName);
   return elements.length > 0 ? elements[0] : null;
 }
 
@@ -104,7 +104,7 @@ export function elementByTag(element, tagName) {
  * @return {?Element}
  */
 export function childElement(parent, callback) {
-  let children = parent.children;
+  const children = parent.children;
   for (let i = 0; i < children.length; i++) {
     if (callback(children[i])) {
       return children[i];
diff --git a/src/event-helper.js b/src/event-helper.js
index 413d6632a1ef..16a9dfe53a9c 100644
--- a/src/event-helper.js
+++ b/src/event-helper.js
@@ -27,9 +27,9 @@ import {timer} from './timer';
  * @return {!Unlisten}
  */
 export function listenOnce(element, eventType, listener, opt_capture) {
-  let capture = opt_capture || false;
+  const capture = opt_capture || false;
   let unlisten;
-  let proxy = event => {
+  const proxy = event => {
     listener(event);
     unlisten();
   };
@@ -54,7 +54,7 @@ export function listenOnce(element, eventType, listener, opt_capture) {
 export function listenOncePromise(element, eventType, opt_capture,
     opt_timeout) {
   let unlisten;
-  let eventPromise = new Promise((resolve, reject) => {
+  const eventPromise = new Promise((resolve, reject) => {
     unlisten = listenOnce(element, eventType, resolve, opt_capture);
   });
   return racePromise_(eventPromise, unlisten, opt_timeout);
@@ -82,7 +82,7 @@ export function isLoaded(element) {
 export function loadPromise(element, opt_timeout) {
   let unlistenLoad;
   let unlistenError;
-  let loadingPromise = new Promise((resolve, reject) => {
+  const loadingPromise = new Promise((resolve, reject) => {
     if (isLoaded(element)) {
       resolve(element);
     } else {
diff --git a/src/focus-history.js b/src/focus-history.js
index e66a22e2cd00..f4ba7ad66881 100644
--- a/src/focus-history.js
+++ b/src/focus-history.js
@@ -66,7 +66,7 @@ export class FocusHistory {
    * @private
    */
   pushFocus_(element) {
-    let now = timer.now();
+    const now = timer.now();
     if (this.history_.length == 0 ||
             this.history_[this.history_.length - 1].el != element) {
       this.history_.push({el: element, time: now});
diff --git a/src/gesture-recognizers.js b/src/gesture-recognizers.js
index 1db6c245a1d2..49a425ea178f 100644
--- a/src/gesture-recognizers.js
+++ b/src/gesture-recognizers.js
@@ -56,7 +56,7 @@ export class TapRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchStart(e) {
-    let touches = e.touches;
+    const touches = e.touches;
     if (!touches || touches.length != 1) {
       return false;
     }
@@ -67,12 +67,12 @@ export class TapRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchMove(e) {
-    let touches = e.changedTouches || e.touches;
+    const touches = e.changedTouches || e.touches;
     if (touches && touches.length == 1) {
       this.lastX_ = touches[0].clientX;
       this.lastY_ = touches[0].clientY;
-      let dx = Math.abs(this.lastX_ - this.startX_) >= 8;
-      let dy = Math.abs(this.lastY_ - this.startY_) >= 8;
+      const dx = Math.abs(this.lastX_ - this.startX_) >= 8;
+      const dy = Math.abs(this.lastY_ - this.startY_) >= 8;
       if (dx || dy) {
         return false;
       }
@@ -137,7 +137,7 @@ export class DoubletapRecognizer extends GestureRecognizer {
     if (this.tapCount_ > 1) {
       return false;
     }
-    let touches = e.touches;
+    const touches = e.touches;
     if (!touches || touches.length != 1) {
       return false;
     }
@@ -148,12 +148,12 @@ export class DoubletapRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchMove(e) {
-    let touches = e.changedTouches || e.touches;
+    const touches = e.changedTouches || e.touches;
     if (touches && touches.length == 1) {
       this.lastX_ = touches[0].clientX;
       this.lastY_ = touches[0].clientY;
-      let dx = Math.abs(this.lastX_ - this.startX_) >= 8;
-      let dy = Math.abs(this.lastY_ - this.startY_) >= 8;
+      const dx = Math.abs(this.lastX_ - this.startX_) >= 8;
+      const dy = Math.abs(this.lastY_ - this.startY_) >= 8;
       if (dx || dy) {
         this.acceptCancel();
         return false;
@@ -259,7 +259,7 @@ class SwipeRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchStart(e) {
-    let touches = e.touches;
+    const touches = e.touches;
     if (!touches || touches.length != 1) {
       return false;
     }
@@ -271,17 +271,17 @@ class SwipeRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchMove(e) {
-    let touches = e.changedTouches || e.touches;
+    const touches = e.changedTouches || e.touches;
     if (touches && touches.length == 1) {
-      let x = touches[0].clientX;
-      let y = touches[0].clientY;
+      const x = touches[0].clientX;
+      const y = touches[0].clientY;
       this.lastX_ = x;
       this.lastY_ = y;
       if (this.eventing_) {
         this.emit_(false, false, e);
       } else {
-        let dx = Math.abs(x - this.startX_);
-        let dy = Math.abs(y - this.startY_);
+        const dx = Math.abs(x - this.startX_);
+        const dy = Math.abs(y - this.startY_);
         // Swipe is penalized slightly since it's one of the least demanding
         // gesture, thus -10 in signalReady.
         if (this.horiz_ && this.vert_) {
@@ -340,7 +340,7 @@ class SwipeRecognizer extends GestureRecognizer {
    */
   emit_(first, last, event) {
     this.lastTime_ = timer.now();
-    let deltaTime = this.lastTime_ - this.prevTime_;
+    const deltaTime = this.lastTime_ - this.prevTime_;
     // It's often that `touchend` arrives on the next frame. These should
     // be ignored to avoid a significant velocity downgrade.
     if (!last && deltaTime > 4 || last && deltaTime > 16) {
@@ -500,7 +500,7 @@ export class TapzoomRecognizer extends GestureRecognizer {
     if (this.eventing_) {
       return false;
     }
-    let touches = e.touches;
+    const touches = e.touches;
     if (!touches || touches.length != 1) {
       return false;
     }
@@ -511,15 +511,15 @@ export class TapzoomRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchMove(e) {
-    let touches = e.changedTouches || e.touches;
+    const touches = e.changedTouches || e.touches;
     if (touches && touches.length == 1) {
       this.lastX_ = touches[0].clientX;
       this.lastY_ = touches[0].clientY;
       if (this.eventing_) {
         this.emit_(false, false, e);
       } else {
-        let dx = Math.abs(this.lastX_ - this.startX_) >= 8;
-        let dy = Math.abs(this.lastY_ - this.startY_) >= 8;
+        const dx = Math.abs(this.lastX_ - this.startX_) >= 8;
+        const dy = Math.abs(this.lastY_ - this.startY_) >= 8;
         if (dx || dy) {
           if (this.tapCount_ == 0) {
             this.acceptCancel();
@@ -690,7 +690,7 @@ export class PinchRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchStart(e) {
-    let touches = e.touches;
+    const touches = e.touches;
     if (!touches || touches.length != 2) {
       return false;
     }
@@ -704,7 +704,7 @@ export class PinchRecognizer extends GestureRecognizer {
 
   /** @override */
   onTouchMove(e) {
-    let touches = e.touches;
+    const touches = e.touches;
     if (!touches || touches.length != 2) {
       return false;
     }
@@ -715,10 +715,10 @@ export class PinchRecognizer extends GestureRecognizer {
     if (this.eventing_) {
       this.emit_(false, false, e);
     } else {
-      let dx1 = this.lastX1_ - this.startX1_;
-      let dy1 = this.lastY1_ - this.startY1_;
-      let dx2 = this.lastX2_ - this.startX2_;
-      let dy2 = this.lastY2_ - this.startY2_;
+      const dx1 = this.lastX1_ - this.startX1_;
+      const dy1 = this.lastY1_ - this.startY1_;
+      const dx2 = this.lastX2_ - this.startX2_;
+      const dy2 = this.lastY2_ - this.startY2_;
       // Fingers should move in opposite directions and go over the threshold.
       if (dx1 * dx2 <= 0 && dy1 * dy2 <= 0) {
         if (Math.abs(dx1 - dx2) >= 8 || Math.abs(dy1 - dy2) >= 8) {
@@ -761,9 +761,9 @@ export class PinchRecognizer extends GestureRecognizer {
    */
   emit_(first, last, event) {
     this.lastTime_ = timer.now();
-    let deltaTime = this.lastTime_ - this.prevTime_;
-    let deltaX = this.deltaX_();
-    let deltaY = this.deltaY_();
+    const deltaTime = this.lastTime_ - this.prevTime_;
+    const deltaX = this.deltaX_();
+    const deltaY = this.deltaY_();
     // It's often that `touchend` arrives on the next frame. These should
     // be ignored to avoid a significant velocity downgrade.
     if (!last && deltaTime > 4 || last && deltaTime > 16) {
@@ -778,9 +778,9 @@ export class PinchRecognizer extends GestureRecognizer {
       this.prevTime_ = this.lastTime_;
     }
 
-    let startSq = this.sqDist_(this.startX1_, this.startX2_,
+    const startSq = this.sqDist_(this.startX1_, this.startX2_,
         this.startY1_, this.startY2_);
-    let lastSq = this.sqDist_(this.lastX1_, this.lastX2_,
+    const lastSq = this.sqDist_(this.lastX1_, this.lastX2_,
         this.lastY1_, this.lastY2_);
     this.signalEmit({
       first: first,
diff --git a/src/gesture.js b/src/gesture.js
index 36c14639b6fc..ab0020344916 100644
--- a/src/gesture.js
+++ b/src/gesture.js
@@ -154,8 +154,8 @@ export class Gestures {
    * @template DATA
    */
   onGesture(recognizerConstr, handler) {
-    let recognizer = new recognizerConstr(this);
-    let type = recognizer.getType();
+    const recognizer = new recognizerConstr(this);
+    const type = recognizer.getType();
     let overserver = this.overservers_[type];
     if (!overserver) {
       this.recognizers_.push(recognizer);
@@ -181,7 +181,7 @@ export class Gestures {
    * @private
    */
   onTouchStart_(event) {
-    let now = timer.now();
+    const now = timer.now();
     this.wasEventing_ = false;
 
     this.pointerDownObservable_.fire(event);
@@ -216,7 +216,7 @@ export class Gestures {
    * @private
    */
   onTouchMove_(event) {
-    let now = timer.now();
+    const now = timer.now();
 
     for (let i = 0; i < this.recognizers_.length; i++) {
       if (!this.tracking_[i]) {
@@ -244,7 +244,7 @@ export class Gestures {
    * @private
    */
   onTouchEnd_(event) {
-    let now = timer.now();
+    const now = timer.now();
 
     for (let i = 0; i < this.recognizers_.length; i++) {
       if (!this.tracking_[i]) {
@@ -295,7 +295,7 @@ export class Gestures {
 
     // Set the recognizer as ready and wait for the pass to
     // make the decision.
-    let now = timer.now();
+    const now = timer.now();
     for (let i = 0; i < this.recognizers_.length; i++) {
       if (this.recognizers_[i] == recognizer) {
         this.ready_[i] = now + offset;
@@ -321,7 +321,7 @@ export class Gestures {
       return;
     }
 
-    let now = timer.now();
+    const now = timer.now();
     for (let i = 0; i < this.recognizers_.length; i++) {
       if (this.recognizers_[i] == recognizer) {
         this.pending_[i] = now + timeLeft;
@@ -353,7 +353,7 @@ export class Gestures {
   signalEmit_(recognizer, data, event) {
     assert(this.eventing_ == recognizer,
         'Recognizer is not currently allowed: %s', recognizer.getType());
-    let overserver = this.overservers_[recognizer.getType()];
+    const overserver = this.overservers_[recognizer.getType()];
     if (overserver) {
       overserver.fire(new Gesture(recognizer.getType(), data, timer.now(),
           event));
@@ -368,7 +368,7 @@ export class Gestures {
     let cancelEvent = !!this.eventing_ || this.wasEventing_;
     this.wasEventing_ = false;
     if (!cancelEvent) {
-      let now = timer.now();
+      const now = timer.now();
       for (let i = 0; i < this.recognizers_.length; i++) {
         if (this.ready_[i] ||
                 this.pending_[i] && this.pending_[i] >= now) {
@@ -394,7 +394,7 @@ export class Gestures {
    * @private
    */
   doPass_() {
-    let now = timer.now();
+    const now = timer.now();
 
     // The "most ready" recognizer is the youngest in the "ready" set.
     // Otherwise we wouldn't wait for it at all.
@@ -442,7 +442,7 @@ export class Gestures {
    * @private
    */
   startEventing_(index) {
-    let recognizer = this.recognizers_[index];
+    const recognizer = this.recognizers_[index];
     for (let i = 0; i < this.recognizers_.length; i++) {
       if (i != index) {
         this.cancelEventing_(i);
diff --git a/src/history.js b/src/history.js
index e4eb52c59989..c3bbb54c0182 100644
--- a/src/history.js
+++ b/src/history.js
@@ -23,11 +23,11 @@ import {viewerFor} from './viewer';
 
 
 /** @private @const */
-let TAG_ = 'History';
+const TAG_ = 'History';
 
 
 /** @private @const */
-let HISTORY_PROP_ = 'AMP.History';
+const HISTORY_PROP_ = 'AMP.History';
 
 
 /**
@@ -35,7 +35,7 @@ let HISTORY_PROP_ = 'AMP.History';
  * @private
  */
 function historyState_(stackIndex) {
-  let state = {};
+  const state = {};
   state[HISTORY_PROP_] = stackIndex;
   return state;
 }
@@ -119,7 +119,7 @@ export class History {
       return;
     }
 
-    let toPop = [];
+    const toPop = [];
     for (let i = this.stackOnPop_.length - 1; i > this.stackIndex_; i--) {
       if (this.stackOnPop_[i]) {
         toPop.push(this.stackOnPop_[i]);
@@ -167,7 +167,7 @@ export class History {
       return;
     }
 
-    let task = this.queue_[0];
+    const task = this.queue_[0];
     let promise;
     try {
       promise = task.callback();
@@ -240,7 +240,7 @@ export class HistoryBindingNatural_ {
     /** @const {!Window} */
     this.win = win;
 
-    let history = this.win.history;
+    const history = this.win.history;
 
     /** @private {number} */
     this.startIndex_ = history.length - 1;
@@ -314,7 +314,7 @@ export class HistoryBindingNatural_ {
     history.pushState = this.historyPushState_.bind(this);
     history.replaceState = this.historyReplaceState_.bind(this);
 
-    let eventPass = new Pass(this.onHistoryEvent_.bind(this), 50);
+    const eventPass = new Pass(this.onHistoryEvent_.bind(this), 50);
     this.popstateHandler_ = e => {
       log.fine(TAG_, 'popstate event: ' + this.win.history.length + ', ' +
           JSON.stringify(e.state));
@@ -380,9 +380,9 @@ export class HistoryBindingNatural_ {
     let state = this.getState_();
     log.fine(TAG_, 'history event: ' + this.win.history.length + ', ' +
         JSON.stringify(state));
-    let stackIndex = state ? state[HISTORY_PROP_] : undefined;
+    const stackIndex = state ? state[HISTORY_PROP_] : undefined;
     let newStackIndex = this.stackIndex_;
-    let waitingState = this.waitingState_;
+    const waitingState = this.waitingState_;
     this.waitingState_ = undefined;
 
     if (newStackIndex > this.win.history.length - 2) {
@@ -461,7 +461,7 @@ export class HistoryBindingNatural_ {
     this.assertReady_();
     let resolve;
     let reject;
-    let promise = timer.timeoutPromise(500,
+    const promise = timer.timeoutPromise(500,
         new Promise((aResolve, aReject) => {
           resolve = aResolve;
           reject = aReject;
@@ -484,7 +484,7 @@ export class HistoryBindingNatural_ {
       return Promise.resolve(this.stackIndex_);
     }
     this.unsupportedState_ = historyState_(this.stackIndex_ - steps);
-    let promise = this.wait_();
+    const promise = this.wait_();
     this.win.history.go(-steps);
     return promise.then(() => {
       return Promise.resolve(this.stackIndex_);
@@ -502,7 +502,7 @@ export class HistoryBindingNatural_ {
     if (!state) {
       state = {};
     }
-    let len = this.win.history.length;
+    const len = this.win.history.length;
     let stackIndex = this.stackIndex_ + 1;
     state[HISTORY_PROP_] = stackIndex;
     this.pushState_(state, title, url);
@@ -525,7 +525,7 @@ export class HistoryBindingNatural_ {
     if (!state) {
       state = {};
     }
-    let stackIndex = Math.min(this.stackIndex_, this.win.history.length - 1);
+    const stackIndex = Math.min(this.stackIndex_, this.win.history.length - 1);
     state[HISTORY_PROP_] = stackIndex;
     this.replaceState_(state, title, url);
     this.updateStackIndex_(stackIndex);
@@ -638,7 +638,7 @@ export class HistoryBindingVirtual_ {
  * @private
  */
 function createHistory_(window) {
-  let viewer = viewerFor(window);
+  const viewer = viewerFor(window);
   let binding;
   if (viewer.isOvertakeHistory()) {
     binding = new HistoryBindingVirtual_(viewer);
diff --git a/src/input.js b/src/input.js
index a5c57ecde1f5..8d61caddf029 100644
--- a/src/input.js
+++ b/src/input.js
@@ -21,10 +21,10 @@ import {listenOnce, listenOncePromise} from './event-helper';
 import {timer} from './timer';
 
 
-let TAG_ = 'Input';
+const TAG_ = 'Input';
 
-let MAX_MOUSE_CONFIRM_ATTEMPS_ = 3;
-let CLICK_TIMEOUT_ = 300;
+const MAX_MOUSE_CONFIRM_ATTEMPS_ = 3;
+const CLICK_TIMEOUT_ = 300;
 
 
 /**
@@ -172,7 +172,7 @@ export class Input {
     }
 
     // Ignore inputs.
-    let target = e.target;
+    const target = e.target;
     if (target && (target.tagName == 'INPUT' ||
           target.tagName == 'TEXTAREA' ||
           target.tagName == 'SELECT' ||
diff --git a/src/layout.js b/src/layout.js
index 6dcdba5076d4..cd483b7009a1 100644
--- a/src/layout.js
+++ b/src/layout.js
@@ -86,7 +86,7 @@ export const LOADING_ELEMENTS_ = {
  *   the layout string.
  */
 export function parseLayout(s) {
-  for (let k in Layout) {
+  for (const k in Layout) {
     if (Layout[k] == s) {
       return Layout[k];
     }
@@ -123,7 +123,7 @@ export function isLayoutSizeDefined(layout) {
  * @return {boolean}
  */
 export function isInternalElement(tag) {
-  let tagName = (typeof tag == 'string') ? tag : tag.tagName;
+  const tagName = (typeof tag == 'string') ? tag : tag.tagName;
   return tagName && tagName.toLowerCase().indexOf('i-') == 0;
 }
 
@@ -170,7 +170,7 @@ export function assertLength(length) {
  */
 export function getLengthUnits(length) {
   assertLength(length);
-  let m = assert(length.match(/[a-z]+/i),
+  const m = assert(length.match(/[a-z]+/i),
       'Failed to read units from %s', length);
   return m[0];
 }
@@ -207,8 +207,8 @@ export function hasNaturalDimensions(tagName) {
 export function getNaturalDimensions(tagName) {
   tagName = tagName.toUpperCase();
   if (!naturalDimensions_[tagName]) {
-    let naturalTagName = tagName.replace(/^AMP\-/, '');
-    let temp = document.createElement(naturalTagName);
+    const naturalTagName = tagName.replace(/^AMP\-/, '');
+    const temp = document.createElement(naturalTagName);
     // For audio, should no-op elsewhere.
     temp.controls = true;
     temp.style.position = 'absolute';
diff --git a/src/loader.js b/src/loader.js
index de8b081c4e3d..7d3f1f80352f 100644
--- a/src/loader.js
+++ b/src/loader.js
@@ -21,10 +21,10 @@
  * @return {!Element}
  */
 export function createLoaderElement() {
-  let loader = document.createElement('div');
+  const loader = document.createElement('div');
   loader.classList.add('-amp-loader');
   for (let i = 0; i < 3; i++) {
-    let dot = document.createElement('div');
+    const dot = document.createElement('div');
     dot.classList.add('-amp-loader-dot');
     loader.appendChild(dot);
   }
diff --git a/src/motion.js b/src/motion.js
index 268eb94c5267..40821925262b 100644
--- a/src/motion.js
+++ b/src/motion.js
@@ -53,7 +53,7 @@ export function calcVelocity(deltaV, deltaTime, prevVelocity) {
   }
 
   // Calculate speed and speed depreciation.
-  let speed = deltaV / deltaTime;
+  const speed = deltaV / deltaTime;
 
   // Depreciation is simply an informational quality. It basically means:
   // we can't ignore the velocity we knew recently, but we'd only consider
@@ -61,7 +61,7 @@ export function calcVelocity(deltaV, deltaTime, prevVelocity) {
   // depreciation factor is 1/100 of a millisecond. New average velocity is
   // calculated by weighing toward the new velocity and away from old
   // velocity based on the depreciation.
-  let depr = 0.5 + Math.min(deltaTime / VELOCITY_DEPR_FACTOR_, 0.5);
+  const depr = 0.5 + Math.min(deltaTime / VELOCITY_DEPR_FACTOR_, 0.5);
   return speed * depr + prevVelocity * (1 - depr);
 }
 
@@ -191,7 +191,7 @@ class Motion {
    * @return {!Promise}
    */
   thenAlways(opt_callback) {
-    let callback = opt_callback || NOOP_CALLBACK_;
+    const callback = opt_callback || NOOP_CALLBACK_;
     return this.then(callback, callback);
   }
 
@@ -202,8 +202,8 @@ class Motion {
   runContinuing_() {
     this.velocityX_ = this.maxVelocityX_;
     this.velocityY_ = this.maxVelocityY_;
-    let boundStep = this.stepContinue_.bind(this);
-    let boundComplete = this.completeContinue_.bind(this, true);
+    const boundStep = this.stepContinue_.bind(this);
+    const boundComplete = this.completeContinue_.bind(this, true);
     return this.vsync_.runMutateSeries(boundStep, 5000)
         .then(boundComplete, boundComplete);
   }
@@ -220,8 +220,8 @@ class Motion {
       return false;
     }
 
-    let prevX = this.lastX_;
-    let prevY = this.lastY_;
+    const prevX = this.lastX_;
+    const prevY = this.lastY_;
 
     this.lastTime_ = timer.now();
     this.lastX_ += timeSincePrev * this.velocityX_;
@@ -230,7 +230,7 @@ class Motion {
       return false;
     }
 
-    let decel = Math.exp(-timeSinceStart / EXP_FRAME_CONST_);
+    const decel = Math.exp(-timeSinceStart / EXP_FRAME_CONST_);
     this.velocityX_ = this.maxVelocityX_ * decel;
     this.velocityY_ = this.maxVelocityY_ * decel;
     return (Math.abs(this.velocityX_) > MIN_VELOCITY_ ||
diff --git a/src/pull-to-refresh.js b/src/pull-to-refresh.js
index fac4d21ec40f..31cebd6420d8 100644
--- a/src/pull-to-refresh.js
+++ b/src/pull-to-refresh.js
@@ -120,7 +120,7 @@ export class PullToRefreshBlocker {
       return;
     }
 
-    let dy = event.touches[0].clientY - this.startPos_;
+    const dy = event.touches[0].clientY - this.startPos_;
 
     // Immediately cancel the P2R if dragging down.
     if (dy > 0) {
diff --git a/src/resources.js b/src/resources.js
index f29d6e0ce40e..6dcfb8d87422 100644
--- a/src/resources.js
+++ b/src/resources.js
@@ -30,18 +30,18 @@ import {viewerFor} from './viewer';
 import {viewportFor} from './viewport';
 import {vsync} from './vsync';
 
-let TAG_ = 'Resources';
-let RESOURCE_PROP_ = '__AMP__RESOURCE';
-let OWNER_PROP_ = '__AMP__OWNER';
-let LAYOUT_TASK_ID_ = 'L';
-let LAYOUT_TASK_OFFSET_ = 0;
-let PRELOAD_TASK_ID_ = 'P';
-let PRELOAD_TASK_OFFSET_ = 2;
-let PRIORITY_BASE_ = 10;
-let PRIORITY_PENALTY_TIME_ = 1000;
-let POST_TASK_PASS_DELAY_ = 1000;
-let MUTATE_DEFER_DELAY_ = 500;
-let FOCUS_HISTORY_TIMEOUT_ = 1000 * 60;  // 1min
+const TAG_ = 'Resources';
+const RESOURCE_PROP_ = '__AMP__RESOURCE';
+const OWNER_PROP_ = '__AMP__OWNER';
+const LAYOUT_TASK_ID_ = 'L';
+const LAYOUT_TASK_OFFSET_ = 0;
+const PRELOAD_TASK_ID_ = 'P';
+const PRELOAD_TASK_OFFSET_ = 2;
+const PRIORITY_BASE_ = 10;
+const PRIORITY_PENALTY_TIME_ = 1000;
+const POST_TASK_PASS_DELAY_ = 1000;
+const MUTATE_DEFER_DELAY_ = 500;
+const FOCUS_HISTORY_TIMEOUT_ = 1000 * 60;  // 1min
 
 
 /**
@@ -192,7 +192,7 @@ export class Resources {
 
   /** @private */
   monitorInput_() {
-    let input = inputFor(this.win);
+    const input = inputFor(this.win);
     input.onTouchDetected(detected => {
       this.toggleInputClass_('amp-mode-touch', detected);
     }, true);
@@ -220,7 +220,7 @@ export class Resources {
     if (!this.win.document.body) {
       return;
     }
-    let scrollHeight = this.win.document.body./*OK*/scrollHeight;
+    const scrollHeight = this.win.document.body./*OK*/scrollHeight;
     if (scrollHeight != this./*OK*/scrollHeight_) {
       this./*OK*/scrollHeight_ = scrollHeight;
       this.viewer_.postDocumentResized(this.viewport_.getSize().width,
@@ -264,7 +264,7 @@ export class Resources {
    * @package
    */
   add(element) {
-    let resource = new Resource((++this.resourceIdCounter_), element, this);
+    const resource = new Resource((++this.resourceIdCounter_), element, this);
     if (!element.id) {
       element.id = 'AMP_' + resource.getId();
     }
@@ -287,8 +287,8 @@ export class Resources {
    * @package
    */
   remove(element) {
-    let resource = this.getResourceForElement(element);
-    let index = resource ? this.resources_.indexOf(resource) : -1;
+    const resource = this.getResourceForElement(element);
+    const index = resource ? this.resources_.indexOf(resource) : -1;
     if (index != -1) {
       this.resources_.splice(index, 1);
     }
@@ -302,7 +302,7 @@ export class Resources {
    * @package
    */
   upgraded(element) {
-    let resource = this.getResourceForElement(element);
+    const resource = this.getResourceForElement(element);
     if (this.isRuntimeOn_) {
       resource.build(this.forceBuild_);
       this.schedulePass();
@@ -441,7 +441,7 @@ export class Resources {
       return;
     }
 
-    let prevVisible = this.visible_;
+    const prevVisible = this.visible_;
     this.visible_ = this.viewer_.isVisible();
     this.prerenderSize_ = this.viewer_.getPrerenderSize();
 
@@ -452,8 +452,8 @@ export class Resources {
       this.updateScrollHeight_();
     }
 
-    let viewportSize = this.viewport_.getSize();
-    let now = timer.now();
+    const viewportSize = this.viewport_.getSize();
+    const now = timer.now();
     log.fine(TAG_, 'PASS: at ' + now +
         ', visible=', this.visible_,
         ', forceBuild=', this.forceBuild_,
@@ -508,16 +508,16 @@ export class Resources {
    */
   mutateWork_() {
     // Read all necessary data before mutates.
-    let now = timer.now();
-    let viewportRect = this.viewport_.getRect();
-    let isScrollingStopped = (Math.abs(this.lastVelocity_) < 1e-2 &&
+    const now = timer.now();
+    const viewportRect = this.viewport_.getRect();
+    const isScrollingStopped = (Math.abs(this.lastVelocity_) < 1e-2 &&
         now - this.lastScrollTime_ > MUTATE_DEFER_DELAY_ ||
         now - this.lastScrollTime_ > MUTATE_DEFER_DELAY_ * 2);
-    let offset = 10;
+    const offset = 10;
 
     if (this.deferredMutates_.length > 0) {
       log.fine(TAG_, 'deferred mutates:', this.deferredMutates_.length);
-      let deferredMutates = this.deferredMutates_;
+      const deferredMutates = this.deferredMutates_;
       this.deferredMutates_ = [];
       for (let i = 0; i < deferredMutates.length; i++) {
         deferredMutates[i]();
@@ -527,15 +527,15 @@ export class Resources {
     if (this.changeHeightRequests_.length > 0) {
       log.fine(TAG_, 'change height requests:',
           this.changeHeightRequests_.length);
-      let changeHeightRequests = this.changeHeightRequests_;
+      const changeHeightRequests = this.changeHeightRequests_;
       this.changeHeightRequests_ = [];
 
       // Find minimum top position and run all mutates.
       let minTop = -1;
       for (let i = 0; i < changeHeightRequests.length; i++) {
-        let request = changeHeightRequests[i];
-        let resource = request.resource;
-        let box = request.resource.getLayoutBox();
+        const request = changeHeightRequests[i];
+        const resource = request.resource;
+        const box = request.resource.getLayoutBox();
         if (box.height == request.newHeight) {
           // Nothing to do.
           continue;
@@ -608,19 +608,19 @@ export class Resources {
 
     // TODO(dvoytenko): vsync separation may be needed for different phases
 
-    let now = timer.now();
+    const now = timer.now();
 
     // Ensure all resources layout phase complete; when relayoutAll is requested
     // force re-layout.
-    let relayoutAll = this.relayoutAll_;
+    const relayoutAll = this.relayoutAll_;
     this.relayoutAll_ = false;
-    let relayoutTop = this.relayoutTop_;
+    const relayoutTop = this.relayoutTop_;
     this.relayoutTop_ = -1;
 
     // Phase 1: Build and relayout as needed. All mutations happen here.
     let relayoutCount = 0;
     for (let i = 0; i < this.resources_.length; i++) {
-      let r = this.resources_[i];
+      const r = this.resources_[i];
       if (r.getState() == ResourceState_.NOT_BUILT) {
         r.build(this.forceBuild_);
       }
@@ -634,7 +634,7 @@ export class Resources {
     // there's no way to optimize this. All reads happen here.
     if (relayoutCount > 0 || relayoutAll || relayoutTop != -1) {
       for (let i = 0; i < this.resources_.length; i++) {
-        let r = this.resources_[i];
+        const r = this.resources_[i];
         if (r.getState() == ResourceState_.NOT_BUILT || r.hasOwner()) {
           continue;
         }
@@ -646,7 +646,7 @@ export class Resources {
       }
     }
 
-    let viewportRect = this.viewport_.getRect();
+    const viewportRect = this.viewport_.getRect();
     // Load viewport = viewport + 3x up/down when document is visible or
     // depending on prerenderSize in pre-render mode.
     let loadRect;
@@ -660,13 +660,13 @@ export class Resources {
     }
 
     // Visible viewport = viewport + 25% up/down.
-    let visibleRect = expandLayoutRect(viewportRect, 0.25, 0.25);
+    const visibleRect = expandLayoutRect(viewportRect, 0.25, 0.25);
 
     // Phase 3: Schedule elements for layout within a reasonable distance from
     // current viewport.
     if (loadRect) {
       for (let i = 0; i < this.resources_.length; i++) {
-        let r = this.resources_[i];
+        const r = this.resources_[i];
         if (r.getState() != ResourceState_.READY_FOR_LAYOUT || r.hasOwner()) {
           continue;
         }
@@ -678,7 +678,7 @@ export class Resources {
 
     // Phase 4: Trigger "viewport enter/exit" events.
     for (let i = 0; i < this.resources_.length; i++) {
-      let r = this.resources_[i];
+      const r = this.resources_[i];
       if (r.hasOwner()) {
         continue;
       }
@@ -699,7 +699,7 @@ export class Resources {
           now > this.exec_.getLastDequeueTime() + 5000) {
       let idleScheduledCount = 0;
       for (let i = 0; i < this.resources_.length; i++) {
-        let r = this.resources_[i];
+        const r = this.resources_[i];
         if (r.getState() == ResourceState_.READY_FOR_LAYOUT &&
                 !r.hasOwner() && r.isDisplayed()) {
           log.fine(TAG_, 'idle layout:', r.debugid);
@@ -720,7 +720,7 @@ export class Resources {
    */
   documentBecameInactive_() {
     for (let i = 0; i < this.resources_.length; i++) {
-      let r = this.resources_[i];
+      const r = this.resources_[i];
       r.documentBecameInactive();
     }
   }
@@ -738,9 +738,9 @@ export class Resources {
    * @private
    */
   work_() {
-    let now = timer.now();
+    const now = timer.now();
 
-    let scorer = this.calcTaskScore_.bind(this, this.viewport_.getRect(),
+    const scorer = this.calcTaskScore_.bind(this, this.viewport_.getRect(),
         Math.sign(this.lastVelocity_));
 
     let timeout = -1;
@@ -760,7 +760,7 @@ export class Resources {
 
         // Do not override a task in execution. This task will have to wait
         // until the current one finished the execution.
-        let executing = this.exec_.getTaskById(task.id);
+        const executing = this.exec_.getTaskById(task.id);
         if (!executing) {
           // Ensure that task can prerender
           task.promise = task.callback(this.visible_);
@@ -820,7 +820,7 @@ export class Resources {
    * @private
    */
   calcTaskScore_(viewportRect, dir, task) {
-    let box = task.resource.getLayoutBox();
+    const box = task.resource.getLayoutBox();
     let posPriority = Math.floor((box.top - viewportRect.top) /
         viewportRect.height);
     if (posPriority != 0 && Math.sign(posPriority) != (dir || 1)) {
@@ -849,12 +849,12 @@ export class Resources {
       return 0;
     }
 
-    let now = timer.now();
+    const now = timer.now();
     let timeout = 0;
     this.exec_.forEach(other => {
       // Higher priority tasks get the head start. Currently 500ms per a drop
       // in priority (note that priority is 10-based).
-      let penalty = Math.max((task.priority - other.priority) *
+      const penalty = Math.max((task.priority - other.priority) *
           PRIORITY_PENALTY_TIME_, 0);
       // TODO(dvoytenko): Consider running total and not maximum.
       timeout = Math.max(timeout, penalty - (now - other.startTime));
@@ -978,7 +978,7 @@ export class Resources {
    * @private
    */
   scheduleLayoutOrPreloadForSubresources_(parentResource, layout, subElements) {
-    let resources = [];
+    const resources = [];
     this.discoverResourcesForArray_(parentResource, subElements, resource => {
       if (resource.getState() != ResourceState_.NOT_BUILT) {
         resources.push(resource);
@@ -1006,9 +1006,9 @@ export class Resources {
    * @private
    */
   schedule_(resource, localId, priorityOffset, parentPriority, callback) {
-    let taskId = resource.debugid + '#' + localId;
+    const taskId = resource.debugid + '#' + localId;
 
-    let task = {
+    const task = {
       id: taskId,
       resource: resource,
       priority: Math.max(resource.getPriority(), parentPriority) +
@@ -1020,7 +1020,7 @@ export class Resources {
 
     // Only schedule a new task if there's no one enqueued yet or if this task
     // has a higher priority.
-    let queued = this.queue_.getTaskById(taskId);
+    const queued = this.queue_.getTaskById(taskId);
     if (!queued || task.priority < queued.priority) {
       if (queued) {
         this.queue_.dequeue(queued);
@@ -1040,7 +1040,7 @@ export class Resources {
    */
   updateInViewportForSubresources_(parentResource, subElements,
       inLocalViewport) {
-    let inViewport = parentResource.isInViewport() && inLocalViewport;
+    const inViewport = parentResource.isInViewport() && inLocalViewport;
     this.discoverResourcesForArray_(parentResource, subElements, resource => {
       resource.setInViewport(inViewport);
     });
@@ -1068,10 +1068,10 @@ export class Resources {
     if (element.classList.contains('-amp-element')) {
       callback(this.getResourceForElement(element));
     } else {
-      let ampElements = element.getElementsByClassName('-amp-element');
-      let seen = [];
+      const ampElements = element.getElementsByClassName('-amp-element');
+      const seen = [];
       for (let i = 0; i < ampElements.length; i++) {
-        let ampElement = ampElements[i];
+        const ampElement = ampElements[i];
         let covered = false;
         for (let j = 0; j < seen.length; j++) {
           if (seen[j].contains(ampElement)) {
@@ -1158,7 +1158,7 @@ export class Resource {
    */
   getOwner() {
     if (this.owner_ === undefined) {
-      let n = this.element;
+      const n = this.element;
       for (let n = this.element; n; n = n.parentElement) {
         if (n[OWNER_PROP_]) {
           this.owner_ = n[OWNER_PROP_];
@@ -1251,7 +1251,7 @@ export class Resource {
       // Can't measure unbuilt element.
       return;
     }
-    let box = this.resources_.viewport_.getLayoutRect(this.element);
+    const box = this.resources_.viewport_.getLayoutRect(this.element);
     // Note that "left" doesn't affect readiness for the layout.
     if (this.state_ == ResourceState_.NOT_LAID_OUT ||
           this.layoutBox_.top != box.top ||
@@ -1557,7 +1557,7 @@ export class TaskQueue_ {
    * @return {boolean}
    */
   dequeue(task) {
-    let existing = this.taskIdMap_[task.id];
+    const existing = this.taskIdMap_[task.id];
     if (!existing) {
       return false;
     }
@@ -1577,8 +1577,8 @@ export class TaskQueue_ {
     let minScore = 1e6;
     let minTask = null;
     for (let i = 0; i < this.tasks_.length; i++) {
-      let task = this.tasks_[i];
-      let score = scorer(task);
+      const task = this.tasks_[i];
+      const score = scorer(task);
       if (score < minScore) {
         minScore = score;
         minTask = task;
diff --git a/src/runtime.js b/src/runtime.js
index 9a110a1e29a9..5684a1103146 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -42,7 +42,7 @@ export function adopt(global) {
   global.AMP_TAG = true;
   // If there is already a global AMP object we assume it is an array
   // of functions
-  let preregisteredElements = global.AMP || [];
+  const preregisteredElements = global.AMP || [];
 
   global.AMP = {};
   /**
@@ -75,7 +75,7 @@ export function adopt(global) {
   /** @const */
   global.AMP.assert = assert;
 
-  let viewer = viewerFor(global);
+  const viewer = viewerFor(global);
 
   /** @const */
   global.AMP.viewer = viewer;
@@ -87,7 +87,7 @@ export function adopt(global) {
     global.AMP.resources = resourcesFor(global);
   }
 
-  let viewport = viewportFor(global);
+  const viewport = viewportFor(global);
 
   /** @const */
   global.AMP.viewport = {};
@@ -106,7 +106,7 @@ export function adopt(global) {
 
   // Execute asynchronously scheduled elements.
   for (let i = 0; i < preregisteredElements.length; i++) {
-    let fn = preregisteredElements[i];
+    const fn = preregisteredElements[i];
     fn(global.AMP);
   }
 }
@@ -121,7 +121,7 @@ export function adopt(global) {
  */
 export function registerForUnitTest(win) {
   for (let i = 0; i < elementsForTesting.length; i++) {
-    let element = elementsForTesting[i];
+    const element = elementsForTesting[i];
     registerElement(win, element.name, element.implementationClass);
   }
 }
diff --git a/src/size-list.js b/src/size-list.js
index 4ad6b14d1211..825b69229c1f 100644
--- a/src/size-list.js
+++ b/src/size-list.js
@@ -42,9 +42,9 @@ var SizeListOption;
  * @return {!SizeList}
  */
 export function parseSizeList(s) {
-  let sSizes = s.split(',');
+  const sSizes = s.split(',');
   assert(sSizes.length > 0, 'sizes has to have at least one size');
-  let sizes = [];
+  const sizes = [];
   sSizes.forEach(sSize => {
     sSize = sSize.replace(/\s+/g, ' ').trim();
     if (sSize.length == 0) {
@@ -53,7 +53,7 @@ export function parseSizeList(s) {
 
     let mediaStr;
     let sizeStr;
-    let spaceIndex = sSize.lastIndexOf(' ');
+    const spaceIndex = sSize.lastIndexOf(' ');
     if (spaceIndex != -1) {
       mediaStr = sSize.substring(0, spaceIndex).trim();
       sizeStr = sSize.substring(spaceIndex + 1).trim();
@@ -88,7 +88,7 @@ export class SizeList {
     // All sources except for last must have a media query. The last one must
     // not.
     for (let i = 0; i < sizes.length; i++) {
-      let option = sizes[i];
+      const option = sizes[i];
       if (i < sizes.length - 1) {
         assert(option.mediaQuery,
             'All options except for the last must have a media condition');
@@ -109,7 +109,7 @@ export class SizeList {
    */
   select(win) {
     for (let i = 0; i < this.sizes_.length - 1; i++) {
-      let option = this.sizes_[i];
+      const option = this.sizes_[i];
       if (win.matchMedia(option.mediaQuery).matches) {
         return option.size;
       }
diff --git a/src/srcset.js b/src/srcset.js
index e3484d5499ed..0d88e2b78bd2 100644
--- a/src/srcset.js
+++ b/src/srcset.js
@@ -41,28 +41,28 @@ export function parseSrcset(s) {
   // Example 1: "image1.png 100w, image2.png 50w"
   // Example 2: "image1.png 2x, image2.png"
   // Example 3: "image1,100w.png 100w, image2.png 50w"
-  let sSources = s.match(
+  const sSources = s.match(
       /\s*([^\s]*)(\s+(-?(\d+(\.(\d+)?)?|\.\d+)[a-zA-Z]))?(\s*,)?/g);
   assert(sSources.length > 0, 'srcset has to have at least one source');
-  let sources = [];
+  const sources = [];
   sSources.forEach(sSource => {
     sSource = sSource.trim();
     if (sSource.substr(-1) == ',') {
       sSource = sSource.substr(0, sSource.length - 1).trim();
     }
-    let parts = sSource.split(/\s+/, 2);
+    const parts = sSource.split(/\s+/, 2);
     if (parts.length == 0 ||
           parts.length == 1 && !parts[0] ||
           parts.length == 2 && !parts[0] && !parts[1]) {
       return;
     }
-    let url = parts[0].trim();
+    const url = parts[0].trim();
     if (parts.length == 1 || parts.length == 2 && !parts[1]) {
       // If no "w" or "x" specified, we assume it's "1x".
       sources.push({url: url, dpr: 1});
     } else {
-      let spec = parts[1].trim().toLowerCase();
-      let lastChar = spec.substring(spec.length - 1);
+      const spec = parts[1].trim().toLowerCase();
+      const lastChar = spec.substring(spec.length - 1);
       if (lastChar == 'w') {
         sources.push({url: url, width: parseFloat(spec)});
       } else if (lastChar == 'x') {
@@ -181,7 +181,7 @@ export class Srcset {
     let minWidth = 1000000;
     let minScore = 1000000;
     for (let i = 0; i < this.sources_.length; i++) {
-      let source = this.sources_[i];
+      const source = this.sources_[i];
       let sourceWidth;
       if (source.width) {
         sourceWidth = source.width / dpr;
@@ -193,7 +193,7 @@ export class Srcset {
       minWidth = Math.min(minWidth, sourceWidth);
       // The calculation is slightly biased toward higher width by offsetting
       // score by negative 0.2.
-      let score = Math.abs((sourceWidth - width) / width - 0.2);
+      const score = Math.abs((sourceWidth - width) / width - 0.2);
       if (score < minScore) {
         minScore = score;
         minIndex = i;
@@ -212,10 +212,10 @@ export class Srcset {
     let minIndex = -1;
     let minScore = 1000000;
     for (let i = 0; i < this.sources_.length; i++) {
-      let source = this.sources_[i];
+      const source = this.sources_[i];
       // Default DPR = 1.
-      let sourceDpr = source.dpr || 1;
-      let score = Math.abs(sourceDpr - dpr);
+      const sourceDpr = source.dpr || 1;
+      const score = Math.abs(sourceDpr - dpr);
       if (score < minScore) {
         minScore = score;
         minIndex = i;
diff --git a/src/style.js b/src/style.js
index 02092b9cb475..e571742252ed 100644
--- a/src/style.js
+++ b/src/style.js
@@ -43,7 +43,7 @@ export function camelCaseToTitleCase(camelCase) {
  */
 function getVendorJsPropertyName_(object, titleCase) {
   for (let i = 0; i < vendorPrefixes_.length; i++) {
-    let propertyName = vendorPrefixes_[i] + titleCase;
+    const propertyName = vendorPrefixes_[i] + titleCase;
     if (object[propertyName] !== undefined) {
       return propertyName;
     }
@@ -67,8 +67,8 @@ export function getVendorJsPropertyName(object, camelCase, opt_bypassCache) {
   if (!propertyName || opt_bypassCache) {
     propertyName = camelCase;
     if (object[camelCase] === undefined) {
-      let titleCase = camelCaseToTitleCase(camelCase);
-      let prefixedPropertyName = getVendorJsPropertyName_(object, titleCase);
+      const titleCase = camelCaseToTitleCase(camelCase);
+      const prefixedPropertyName = getVendorJsPropertyName_(object, titleCase);
 
       if (object[prefixedPropertyName] !== undefined) {
         propertyName = prefixedPropertyName;
@@ -91,7 +91,7 @@ export function getVendorJsPropertyName(object, camelCase, opt_bypassCache) {
  * @param {boolean=} opt_bypassCache
  */
 export function setStyle(element, property, value, opt_units, opt_bypassCache) {
-  let propertyName = getVendorJsPropertyName(element.style, property,
+  const propertyName = getVendorJsPropertyName(element.style, property,
       opt_bypassCache);
   if (propertyName) {
     element.style[propertyName] = opt_units ? value + opt_units : value;
@@ -107,7 +107,7 @@ export function setStyle(element, property, value, opt_units, opt_bypassCache) {
  * @return {*}
  */
 export function getStyle(element, property, opt_bypassCache) {
-  let propertyName = getVendorJsPropertyName(element.style, property,
+  const propertyName = getVendorJsPropertyName(element.style, property,
       opt_bypassCache);
   if (!propertyName) {
     return undefined;
@@ -123,7 +123,7 @@ export function getStyle(element, property, opt_bypassCache) {
  * @param {!Object<string, *>} styles
  */
 export function setStyles(element, styles) {
-  for (let k in styles) {
+  for (const k in styles) {
     setStyle(element, k, styles[k]);
   }
 }
diff --git a/src/styles.js b/src/styles.js
index b433449198b8..3c7ce60e6e8b 100644
--- a/src/styles.js
+++ b/src/styles.js
@@ -80,7 +80,7 @@ export function installStyles(doc, cssText, cb, opt_isRuntimeCss) {
  */
 export function makeBodyVisible(doc) {
   let interval;
-  let set = () => {
+  const set = () => {
     if (doc.body) {
       doc.body.style.opacity = 1;
       clearInterval(interval);
diff --git a/src/swipe.js b/src/swipe.js
index ced910d5e1e4..37c73db37869 100644
--- a/src/swipe.js
+++ b/src/swipe.js
@@ -246,7 +246,7 @@ export class SwipeXRecognizer {
       this.velocity_ = 0;
       this.end_(/* allowContinuing */ false);
     }
-    let touches = e.changedTouches || e.touches;
+    const touches = e.changedTouches || e.touches;
     if (touches && touches.length == 1) {
       this.tracking_ = true;
       this.startX_ = touches[0].clientX;
@@ -262,10 +262,10 @@ export class SwipeXRecognizer {
     if (!this.tracking_) {
       return;
     }
-    let touches = e.changedTouches || e.touches;
+    const touches = e.changedTouches || e.touches;
     if (touches && touches.length > 0) {
-      let x = touches[0].clientX;
-      let y = touches[0].clientY;
+      const x = touches[0].clientX;
+      const y = touches[0].clientY;
       if (this.eventing_) {
         this.move_(x);
       } else {
@@ -286,7 +286,7 @@ export class SwipeXRecognizer {
     if (!this.tracking_) {
       return;
     }
-    let touches = e.changedTouches || e.touches;
+    const touches = e.changedTouches || e.touches;
     if (this.eventing_ && touches && touches.length > 0) {
       if (this.lastX != touches[0].clientX) {
         this.move_(touches[0].clientX);
@@ -324,8 +324,8 @@ export class SwipeXRecognizer {
    * @private
    */
   move_(x) {
-    let prevX = this.lastX_;
-    let prevTime = this.lastTime_;
+    const prevX = this.lastX_;
+    const prevTime = this.lastTime_;
     this.lastX_ = x;
     this.lastTime_ = timer.now();
     if (this.lastTime_ - prevTime > 2) {
@@ -341,7 +341,7 @@ export class SwipeXRecognizer {
    * @private
    */
   fireMove_(synthetic, continued) {
-    let delta = this.calcDelta_(this.startX_, this.lastX_);
+    const delta = this.calcDelta_(this.startX_, this.lastX_);
     this.moveObservable_.fire({
       delta: delta,
       velocity: this.velocity_,
@@ -364,7 +364,7 @@ export class SwipeXRecognizer {
     if (!this.eventing_) {
       return;
     }
-    let endTime = timer.now();
+    const endTime = timer.now();
     if (endTime - this.lastTime_ > 2) {
       this.velocity_ = this.calcVelocity_(this.lastX_, this.lastX_,
           this.lastTime_, endTime, this.velocity_);
@@ -381,7 +381,7 @@ export class SwipeXRecognizer {
   endFinal_() {
     this.continuing_ = false;
     this.eventing_ = false;
-    let delta = this.calcDelta_(this.startX_, this.lastX_);
+    const delta = this.calcDelta_(this.startX_, this.lastX_);
     this.endObservable_.fire({
       delta: delta,
       velocity: this.velocity_,
@@ -397,28 +397,28 @@ export class SwipeXRecognizer {
   startContinuing_() {
     this.continuing_ = true;
 
-    let delta = this.calcDelta_(this.startX_, this.lastX_);
+    const delta = this.calcDelta_(this.startX_, this.lastX_);
 
     // 1. The user overpulls - spring back
     if (delta > this.maxDelta_ || delta < this.minDelta_) {
-      let retDelta = delta > this.maxDelta_ ? this.maxDelta_ - delta :
+      const retDelta = delta > this.maxDelta_ ? this.maxDelta_ - delta :
           this.minDelta_ - delta;
       // Reverse the velocity.
-      let maxVelocity = Math.sign(retDelta) *
+      const maxVelocity = Math.sign(retDelta) *
           Math.max(Math.abs(this.velocity_) * 0.8, 0.25);
       this.lastX_ = this.startX_ + delta;
-      let endX = this.lastX_ + retDelta;
-      let overshoot = Math.abs(retDelta) * maxVelocity;
-      let spring = tr.spring(this.lastX_, endX, endX + overshoot, 0.8);
+      const endX = this.lastX_ + retDelta;
+      const overshoot = Math.abs(retDelta) * maxVelocity;
+      const spring = tr.spring(this.lastX_, endX, endX + overshoot, 0.8);
       return this.runContinuing_(maxVelocity, false, velocity => {
-        let vnorm = 1 - Math.abs(velocity / maxVelocity);
+        const vnorm = 1 - Math.abs(velocity / maxVelocity);
         return spring(vnorm);
       });
     }
 
     // 2. Pull to one of the boundaries
     if (this.snapPoint_) {
-      let snapPoint = Math.abs(this.snapPoint_ / this.positionMultiplier_);
+      const snapPoint = Math.abs(this.snapPoint_ / this.positionMultiplier_);
       let newDelta = 0;
       if (delta < 0 && Math.abs(delta) > snapPoint ||
               this.velocity_ < -0.1) {
@@ -431,13 +431,13 @@ export class SwipeXRecognizer {
       }
       if (newDelta != delta) {
         // Reverse the velocity.
-        let maxVelocity = Math.sign(newDelta - delta) *
+        const maxVelocity = Math.sign(newDelta - delta) *
             Math.max(Math.abs(this.velocity_) * 0.95, 0.5);
         this.lastX_ = this.startX_ + delta;
-        let endX = this.startX_ + newDelta;
-        let func = tr.numeric(this.lastX_, endX);
+        const endX = this.startX_ + newDelta;
+        const func = tr.numeric(this.lastX_, endX);
         return this.runContinuing_(maxVelocity, false, velocity => {
-          let vnorm = 1 - Math.abs(velocity / maxVelocity);
+          const vnorm = 1 - Math.abs(velocity / maxVelocity);
           return func(vnorm);
         });
       }
@@ -445,7 +445,7 @@ export class SwipeXRecognizer {
 
     // Intertia.
     if (Math.abs(this.velocity_) > MIN_VELOCITY_) {
-      let maxVelocity = this.velocity_ * 0.95;  // First exponential order
+      const maxVelocity = this.velocity_ * 0.95;  // First exponential order
       return this.runContinuing_(maxVelocity, true,
           (velocity, timeSinceStart, timeSincePrev) => {
             return this.lastX_ + timeSincePrev * velocity;
@@ -472,7 +472,7 @@ export class SwipeXRecognizer {
     }
 
     this.velocity_ = maxVelocity;
-    let completeContinue = () => {
+    const completeContinue = () => {
       this.lastTime_ = timer.now();
       this.lastX_ = velocityFunc(0, 0, 0);
       this.fireMove_(/* synthetic */ true, /* continued */ true);
@@ -490,7 +490,7 @@ export class SwipeXRecognizer {
       this.lastX_ = velocityFunc(this.velocity_, timeSinceStart,
           timeSincePrev);
       this.fireMove_(/* synthetic */ true, /* continued */ true);
-      let prevVelocity = this.velocity_;
+      const prevVelocity = this.velocity_;
       this.velocity_ = maxVelocity * Math.exp(-timeSinceStart / FRAME_CONST_);
       return Math.abs(this.velocity_) > MIN_VELOCITY_;
     }, 5000).then(completeContinue, completeContinue);
@@ -515,14 +515,14 @@ export class SwipeXRecognizer {
    * @private
    */
   calcVelocity_(prevX, newX, prevTime, newTime, prevVelocity) {
-    let dx = newX - prevX;
-    let dt = newTime - prevTime;
+    const dx = newX - prevX;
+    const dt = newTime - prevTime;
     if (Math.abs(dt) < 2) {
       return 0;
     }
 
     // Calculate speed and speed depreciation.
-    let speed = dx / dt;
+    const speed = dx / dt;
 
     // Depreciation is simply an informational quality. It basically means:
     // we can't ignore the velocity we knew recently, but we'd only consider
@@ -530,8 +530,8 @@ export class SwipeXRecognizer {
     // depreciation factor is 1/100 of a millisecond. New average velocity is
     // calculated by weighing toward the new velocity and away from old
     // velocity based on the depreciation.
-    let deprFactor = 100;
-    let depr = 0.5 + Math.min(dt / deprFactor, 0.5);
+    const deprFactor = 100;
+    const depr = 0.5 + Math.min(dt / deprFactor, 0.5);
     return speed * depr + prevVelocity * (1 - depr);
   }
 }
diff --git a/src/transition.js b/src/transition.js
index e5639930e03d..194a9a63417a 100644
--- a/src/transition.js
+++ b/src/transition.js
@@ -42,7 +42,7 @@ export const NOOP = function(time) {return null;};
 export function all(transitions) {
   return (time, complete) => {
     for (let i = 0; i < transitions.length; i++) {
-      let tr = transitions[i];
+      const tr = transitions[i];
       tr(time, complete);
     }
   };
@@ -75,7 +75,7 @@ export function withCurve(transition, curve) {
  */
 export function setStyles(element, styles) {
   return (time, complete) => {
-    for (let k in styles) {
+    for (const k in styles) {
       st.setStyle(element, k, styles[k](time, complete));
     }
   };
@@ -138,7 +138,7 @@ export function px(transition) {
  */
 export function translateX(transition) {
   return time => {
-    let res = transition(time);
+    const res = transition(time);
     if (typeof res == 'string') {
       return `translateX(${res})`;
     }
diff --git a/src/url.js b/src/url.js
index a856ac62c332..e85edce46650 100644
--- a/src/url.js
+++ b/src/url.js
@@ -81,10 +81,10 @@ export function parseQueryString(queryString) {
   if (queryString.indexOf('?') == 0 || queryString.indexOf('#') == 0) {
     queryString = queryString.substr(1);
   }
-  let pairs = queryString.split('&');
+  const pairs = queryString.split('&');
   for (let i = 0; i < pairs.length; i++) {
-    let pair = pairs[i];
-    let eqIndex = pair.indexOf('=');
+    const pair = pairs[i];
+    const eqIndex = pair.indexOf('=');
     let name;
     let value;
     if (eqIndex != -1) {
@@ -125,7 +125,7 @@ export function getOrigin(info) {
  * @return {string}
  */
 export function removeFragment(url) {
-  let index = url.indexOf('#');
+  const index = url.indexOf('#');
   if (index == -1) {
     return url;
   }
diff --git a/src/viewer.js b/src/viewer.js
index 4178e52ceff5..ee5fe490c6ef 100644
--- a/src/viewer.js
+++ b/src/viewer.js
@@ -23,8 +23,8 @@ import {parseQueryString, removeFragment} from './url';
 import {platform} from './platform';
 
 
-let TAG_ = 'Viewer';
-let SENTINEL_ = '__AMP__';
+const TAG_ = 'Viewer';
+const SENTINEL_ = '__AMP__';
 
 
 /**
@@ -478,7 +478,7 @@ export class Viewer {
     assert(!this.messageDeliverer_, 'message deliverer can only be set once');
     this.messageDeliverer_ = deliverer;
     if (this.messageQueue_.length > 0) {
-      let queue = this.messageQueue_.slice(0);
+      const queue = this.messageQueue_.slice(0);
       this.messageQueue_ = [];
       queue.forEach(message => {
         this.messageDeliverer_(message.eventType, message.data, false);
@@ -531,8 +531,8 @@ export class Viewer {
  * @private
  */
 export function parseParams_(str, allParams) {
-  let params = parseQueryString(str);
-  for (let k in params) {
+  const params = parseQueryString(str);
+  for (const k in params) {
     allParams[k] = params[k];
   }
 }
diff --git a/src/viewport.js b/src/viewport.js
index 33d2dcd0f7b2..c40aab71782c 100644
--- a/src/viewport.js
+++ b/src/viewport.js
@@ -26,7 +26,7 @@ import {timer} from './timer';
 import {viewerFor} from './viewer';
 
 
-let TAG_ = 'Viewport';
+const TAG_ = 'Viewport';
 
 
 /**
@@ -100,7 +100,7 @@ export class Viewport {
 
     this.viewer_.onViewportEvent(() => {
       this.binding_.updateViewerViewport(this.viewer_);
-      let paddingTop = this.viewer_.getPaddingTop();
+      const paddingTop = this.viewer_.getPaddingTop();
       if (paddingTop != this.paddingTop_) {
         this.paddingTop_ = paddingTop;
         this.binding_.updatePaddingTop(this.paddingTop_);
@@ -220,8 +220,8 @@ export class Viewport {
    * @param {!Element} element
    */
   scrollIntoView(element) {
-    let elementTop = this.binding_.getLayoutRect(element).top;
-    let newScrollTop = Math.max(0, elementTop - this.paddingTop_);
+    const elementTop = this.binding_.getLayoutRect(element).top;
+    const newScrollTop = Math.max(0, elementTop - this.paddingTop_);
     this.binding_.setScrollTop(newScrollTop);
   }
 
@@ -251,8 +251,9 @@ export class Viewport {
    * Resets touch zoom to initial scale of 1.
    */
   resetTouchZoom() {
-    let windowHeight = this.win_./*OK*/innerHeight;
-    let documentHeight = this.win_.document.documentElement./*OK*/clientHeight;
+    const windowHeight = this.win_./*OK*/innerHeight;
+    const documentHeight = this.win_.document
+        .documentElement./*OK*/clientHeight;
     if (windowHeight && documentHeight && windowHeight === documentHeight) {
       // This code only works when scrollbar overlay content and take no space,
       // which is fine on mobile. For non-mobile devices this code is
@@ -272,14 +273,14 @@ export class Viewport {
    * @return {boolean}
    */
   disableTouchZoom() {
-    let viewportMeta = this.getViewportMeta_();
+    const viewportMeta = this.getViewportMeta_();
     if (!viewportMeta) {
       // This should never happen in a valid AMP document, thus shortcircuit.
       return false;
     }
     // Setting maximum-scale=1 and user-scalable=no zooms page back to normal
     // and prohibit further default zooming.
-    let newValue = updateViewportMetaString(viewportMeta.content, {
+    const newValue = updateViewportMetaString(viewportMeta.content, {
       'maximum-scale': '1',
       'user-scalable': 'no'
     });
@@ -304,7 +305,7 @@ export class Viewport {
    * @return {boolean}
    */
   setViewportMetaString_(viewportMetaString) {
-    let viewportMeta = this.getViewportMeta_();
+    const viewportMeta = this.getViewportMeta_();
     if (viewportMeta && viewportMeta.content != viewportMetaString) {
       log.fine(TAG_, 'changed viewport meta to:', viewportMetaString);
       viewportMeta.content = viewportMetaString;
@@ -338,8 +339,8 @@ export class Viewport {
    * @private
    */
   changed_(relayoutAll, velocity) {
-    let size = this.getSize();
-    let scrollTop = this.getScrollTop();
+    const size = this.getSize();
+    const scrollTop = this.getScrollTop();
     log.fine(TAG_, 'changed event:',
         'relayoutAll=', relayoutAll,
         'top=', scrollTop,
@@ -414,9 +415,9 @@ export class Viewport {
 
   /** @private */
   resize_() {
-    let oldSize = this.size_;
+    const oldSize = this.size_;
     this.size_ = null;  // Need to recalc.
-    let newSize = this.getSize();
+    const newSize = this.getSize();
     this.changed_(!oldSize || oldSize.width != newSize.width, 0);
   }
 }
@@ -668,8 +669,8 @@ export class ViewportBindingNaturalIosEmbed_ {
 
   /** @private */
   setup_() {
-    let documentElement = this.win.document.documentElement;
-    let documentBody = this.win.document.body;
+    const documentElement = this.win.document.documentElement;
+    const documentBody = this.win.document.body;
 
     // TODO(dvoytenko): need to also find a way to do this on resize.
     this.scrollWidth_ = documentBody./*OK*/scrollWidth || 0;
@@ -821,7 +822,7 @@ export class ViewportBindingNaturalIosEmbed_ {
       return;
     }
     this.adjustScrollPos_(event);
-    let rect = this.scrollPosEl_./*OK*/getBoundingClientRect();
+    const rect = this.scrollPosEl_./*OK*/getBoundingClientRect();
     if (this.pos_.x != -rect.left || this.pos_.y != -rect.top) {
       this.pos_.x = -rect.left;
       this.pos_.y = -rect.top;
@@ -850,7 +851,7 @@ export class ViewportBindingNaturalIosEmbed_ {
     // Scroll document into a safe position to avoid scroll freeze on iOS.
     // This means avoiding scrollTop to be minimum (0) or maximum value.
     // This is very sad but very necessary. See #330 for more details.
-    let scrollTop = -this.scrollPosEl_./*OK*/getBoundingClientRect().top;
+    const scrollTop = -this.scrollPosEl_./*OK*/getBoundingClientRect().top;
     if (scrollTop == 0) {
       this.setScrollPos_(1);
       if (opt_event) {
@@ -989,15 +990,15 @@ export class ViewportBindingVirtual_ {
  */
 export function parseViewportMeta(content) {
   // Ex: width=device-width,initial-scale=1,minimal-ui
-  let params = Object.create(null);
+  const params = Object.create(null);
   if (!content) {
     return params;
   }
-  let pairs = content.split(',');
+  const pairs = content.split(',');
   for (let i = 0; i < pairs.length; i++) {
-    let pair = pairs[i];
-    let split = pair.split('=');
-    let name = split[0].trim();
+    const pair = pairs[i];
+    const split = pair.split('=');
+    const name = split[0].trim();
     let value = split[1];
     value = (value || '').trim();
     if (name) {
@@ -1021,7 +1022,7 @@ export function parseViewportMeta(content) {
 export function stringifyViewportMeta(params) {
   // Ex: width=device-width,initial-scale=1,minimal-ui
   let content = '';
-  for (let k in params) {
+  for (const k in params) {
     if (content.length > 0) {
       content += ',';
     }
@@ -1046,9 +1047,9 @@ export function stringifyViewportMeta(params) {
  * @private Visible for testing only.
  */
 export function updateViewportMetaString(currentValue, updateParams) {
-  let params = parseViewportMeta(currentValue);
+  const params = parseViewportMeta(currentValue);
   let changed = false;
-  for (let k in updateParams) {
+  for (const k in updateParams) {
     if (params[k] !== updateParams[k]) {
       changed = true;
       if (updateParams[k] !== undefined) {
@@ -1071,7 +1072,7 @@ export function updateViewportMetaString(currentValue, updateParams) {
  * @private
  */
 function createViewport_(window) {
-  let viewer = viewerFor(window);
+  const viewer = viewerFor(window);
   let binding;
   if (viewer.getViewportType() == 'virtual') {
     binding = new ViewportBindingVirtual_(window, viewer);
diff --git a/src/vsync.js b/src/vsync.js
index 0ebc760fae00..71429931e67b 100644
--- a/src/vsync.js
+++ b/src/vsync.js
@@ -51,7 +51,7 @@ export class Vsync {
    * @param {!Object<string, *>|undefined} opt_state
    */
   run(task, opt_state) {
-    let state = opt_state || {};
+    const state = opt_state || {};
     this.win.requestAnimationFrame(() => {
       if (task.measure) {
         task.measure(state);
@@ -93,12 +93,12 @@ export class Vsync {
    */
   runMutateSeries(mutator, opt_timeout) {
     return new Promise((resolve, reject) => {
-      let startTime = timer.now();
+      const startTime = timer.now();
       let prevTime = 0;
-      let task = this.createTask({
+      const task = this.createTask({
         mutate: state => {
-          let timeSinceStart = timer.now() - startTime;
-          let res = mutator(timeSinceStart, timeSinceStart - prevTime, state);
+          const timeSinceStart = timer.now() - startTime;
+          const res = mutator(timeSinceStart, timeSinceStart - prevTime, state);
           if (!res) {
             resolve();
           } else if (opt_timeout && timeSinceStart > opt_timeout) {
diff --git a/test/functional/test-3p-frame.js b/test/functional/test-3p-frame.js
index 76b74628373c..02494b50407c 100644
--- a/test/functional/test-3p-frame.js
+++ b/test/functional/test-3p-frame.js
@@ -25,7 +25,7 @@ describe('3p-frame', () => {
   afterEach(() => {
     resetServiceForTesting(window, 'bootstrapBaseUrl');
     setModeForTesting(null);
-    let m = document.querySelector(
+    const m = document.querySelector(
         '[name="amp-3p-iframe-src"]');
     if (m) {
       m.parentElement.removeChild(m);
@@ -33,7 +33,7 @@ describe('3p-frame', () => {
   });
 
   function addCustomBootstrap(url) {
-    let meta = document.createElement('meta');
+    const meta = document.createElement('meta');
     meta.setAttribute('name', 'amp-3p-iframe-src');
     meta.setAttribute('content', url);
     document.head.appendChild(meta);
diff --git a/test/functional/test-action.js b/test/functional/test-action.js
index 2331abe65c41..4526bd23aa05 100644
--- a/test/functional/test-action.js
+++ b/test/functional/test-action.js
@@ -35,33 +35,33 @@ describe('Action parseAction', () => {
 
 
   it('should fail parse without event', () => {
-    let a = action.parseAction_('target1.method1');
+    const a = action.parseAction_('target1.method1');
     expect(a).to.equal(null);
   });
 
   it('should parse with default method', () => {
-    let a = action.parseAction_('event1:target1');
+    const a = action.parseAction_('event1:target1');
     expect(a.event).to.equal('event1');
     expect(a.target).to.equal('target1');
     expect(a.method).to.equal('activate');
   });
 
   it('should parse full form', () => {
-    let a = action.parseAction_('event1:target1.method1');
+    const a = action.parseAction_('event1:target1.method1');
     expect(a.event).to.equal('event1');
     expect(a.target).to.equal('target1');
     expect(a.method).to.equal('method1');
   });
 
   it('should parse with lots of whitespace', () => {
-    let a = action.parseAction_('  event1  :  target1  .  method1  ');
+    const a = action.parseAction_('  event1  :  target1  .  method1  ');
     expect(a.event).to.equal('event1');
     expect(a.target).to.equal('target1');
     expect(a.method).to.equal('method1');
   });
 
   it('should parse empty to null', () => {
-    let a = action.parseAction_('');
+    const a = action.parseAction_('');
     expect(a).to.equal(null);
   });
 
@@ -72,7 +72,7 @@ describe('Action parseAction', () => {
   });
 
   it('should parse with period in event or method', () => {
-    let a = action.parseAction_('event.1:target1.method.1');
+    const a = action.parseAction_('event.1:target1.method.1');
     expect(a.event).to.equal('event.1');
     expect(a.target).to.equal('target1');
     expect(a.method).to.equal('method.1');
@@ -97,18 +97,18 @@ describe('Action parseActionMap', () => {
   });
 
   it('should parse with a single action', () => {
-    let m = action.parseActionMap_('event1:action1');
+    const m = action.parseActionMap_('event1:action1');
     expect(m['event1'].target).to.equal('action1');
   });
 
   it('should parse with two actions', () => {
-    let m = action.parseActionMap_('event1:action1; event2: action2');
+    const m = action.parseActionMap_('event1:action1; event2: action2');
     expect(m['event1'].target).to.equal('action1');
     expect(m['event2'].target).to.equal('action2');
   });
 
   it('should parse with dupe actions by overriding with last', () => {
-    let m = action.parseActionMap_('event1:action1; event1: action2');
+    const m = action.parseActionMap_('event1:action1; event1: action2');
     // Currently, we overwrite the events.
     expect(m['event1'].target).to.equal('action2');
   });
@@ -140,15 +140,15 @@ describe('Action findAction', () => {
   it('should create action map in getActionMap_', () => {
     var element = document.createElement('div');
     element.setAttribute('on', 'event1:action1');
-    let m = action.getActionMap_(element);
+    const m = action.getActionMap_(element);
     expect(m['event1'].target).to.equal('action1');
   });
 
   it('should cache action map', () => {
     var element = document.createElement('div');
     element.setAttribute('on', 'event1:action1');
-    let m1 = action.getActionMap_(element);
-    let m2 = action.getActionMap_(element);
+    const m1 = action.getActionMap_(element);
+    const m2 = action.getActionMap_(element);
     expect(m1).to.equal(m2);
   });
 
@@ -156,7 +156,7 @@ describe('Action findAction', () => {
   it('should find action on the same element', () => {
     var element = document.createElement('div');
     element.setAttribute('on', 'event1:action1');
-    let a = action.findAction_(element, 'event1');
+    const a = action.findAction_(element, 'event1');
     expect(a.node).to.equal(element);
     expect(a.actionInfo.target).to.equal('action1');
 
@@ -195,7 +195,7 @@ describe('Action method', () => {
     action = new Action(window);
     onEnqueue = sinon.spy();
     targetElement = document.createElement('target');
-    let id = ('E' + Math.random()).replace('.', '');
+    const id = ('E' + Math.random()).replace('.', '');
     targetElement.setAttribute('on', 'tap:' + id + '.method1');
     parent = document.createElement('parent');
     child = document.createElement('child');
@@ -220,7 +220,7 @@ describe('Action method', () => {
   it('should invoke on the AMP element', () => {
     action.invoke_(execElement, 'method1', 'source1', 'event1');
     expect(onEnqueue.callCount).to.equal(1);
-    let inv = onEnqueue.getCall(0).args[0];
+    const inv = onEnqueue.getCall(0).args[0];
     expect(inv.target).to.equal(execElement);
     expect(inv.method).to.equal('method1');
     expect(inv.source).to.equal('source1');
@@ -244,7 +244,7 @@ describe('Action method', () => {
   it('should trigger event', () => {
     action.trigger(child, 'tap', null);
     expect(onEnqueue.callCount).to.equal(1);
-    let inv = onEnqueue.getCall(0).args[0];
+    const inv = onEnqueue.getCall(0).args[0];
     expect(inv.target).to.equal(execElement);
     expect(inv.method).to.equal('method1');
     expect(inv.source).to.equal(targetElement);
@@ -253,7 +253,7 @@ describe('Action method', () => {
   it('should execute method', () => {
     action.execute(execElement, 'method1', child, null);
     expect(onEnqueue.callCount).to.equal(1);
-    let inv = onEnqueue.getCall(0).args[0];
+    const inv = onEnqueue.getCall(0).args[0];
     expect(inv.target).to.equal(execElement);
     expect(inv.method).to.equal('method1');
     expect(inv.source).to.equal(child);
diff --git a/test/functional/test-amp-ad.js b/test/functional/test-amp-ad.js
index 8ee6a9f7ea8b..042eca5b3662 100644
--- a/test/functional/test-amp-ad.js
+++ b/test/functional/test-amp-ad.js
@@ -147,16 +147,16 @@ describe('amp-ad', () => {
   describe('scoreDimensions_', () => {
 
     it('should choose a matching dimension', () => {
-      let dims = [[320, 200], [320, 210], [320, 200]];
-      let scores = scoreDimensions_(dims, 320, 200);
-      let winner = scores.indexOf(Math.max.apply(Math, scores));
+      const dims = [[320, 200], [320, 210], [320, 200]];
+      const scores = scoreDimensions_(dims, 320, 200);
+      const winner = scores.indexOf(Math.max.apply(Math, scores));
       expect(winner).to.equal(0);
     });
 
     it('should be biased to a smaller height delta', () => {
-      let dims = [[300, 200], [320, 50]];
-      let scores = scoreDimensions_(dims, 300, 50);
-      let winner = scores.indexOf(Math.max.apply(Math, scores));
+      const dims = [[300, 200], [320, 50]];
+      const scores = scoreDimensions_(dims, 300, 50);
+      const winner = scores.indexOf(Math.max.apply(Math, scores));
       expect(winner).to.equal(1);
     });
   });
diff --git a/test/functional/test-amp-img.js b/test/functional/test-amp-img.js
index 3ef97259202f..852e1119e357 100644
--- a/test/functional/test-amp-img.js
+++ b/test/functional/test-amp-img.js
@@ -38,13 +38,13 @@ describe('amp-img', () => {
         .returns(true);
     return createIframePromise().then(iframe => {
       installImg(iframe.win);
-      let img = iframe.doc.createElement('amp-img');
-      for (let key in attributes) {
+      const img = iframe.doc.createElement('amp-img');
+      for (const key in attributes) {
         img.setAttribute(key, attributes[key]);
       }
 
       if (children != null) {
-        for (let key in children) {
+        for (const key in children) {
           img.appendChild(children[key]);
         }
       }
@@ -58,7 +58,7 @@ describe('amp-img', () => {
       width: 300,
       height: 200
     }).then(ampImg => {
-      let img = ampImg.querySelector('img');
+      const img = ampImg.querySelector('img');
       expect(img).to.be.an.instanceof(Element);
       expect(img.tagName).to.equal('IMG');
       expect(img.getAttribute('src')).to.equal('test.jpg');
diff --git a/test/functional/test-amp-pixel.js b/test/functional/test-amp-pixel.js
index 13330cfb49be..af74f27cbb55 100644
--- a/test/functional/test-amp-pixel.js
+++ b/test/functional/test-amp-pixel.js
@@ -121,7 +121,7 @@ describe('amp-pixel', () => {
         'https://foo.com?ref=$AMPDOC_URL'
         ).then(p => {
           expect(p.querySelector('img')).to.not.be.null;
-          let query = parseQueryString(parseUrl(p.children[0].src).search);
+          const query = parseQueryString(parseUrl(p.children[0].src).search);
           expect(query['ref']).to.not.equal('$AMPDOC_URL');
         });
   });
@@ -131,7 +131,7 @@ describe('amp-pixel', () => {
         'https://foo.com?ref=$AMPDOC_HOST'
         ).then(p => {
           expect(p.querySelector('img')).to.not.be.null;
-          let query = parseQueryString(parseUrl(p.children[0].src).search);
+          const query = parseQueryString(parseUrl(p.children[0].src).search);
           expect(query['ref']).to.not.equal('$AMPDOC_HOST');
         });
   });
diff --git a/test/functional/test-amp-video.js b/test/functional/test-amp-video.js
index 6b619ea839b4..866c5399c5b3 100644
--- a/test/functional/test-amp-video.js
+++ b/test/functional/test-amp-video.js
@@ -27,11 +27,11 @@ describe('amp-video', () => {
     return createIframePromise().then(iframe => {
       installVideo(iframe.win);
       var v = iframe.doc.createElement('amp-video');
-      for (let key in attributes) {
+      for (const key in attributes) {
         v.setAttribute(key, attributes[key]);
       }
       if (children != null) {
-        for (let key in children) {
+        for (const key in children) {
           v.appendChild(children[key]);
         }
       }
diff --git a/test/functional/test-animation.js b/test/functional/test-animation.js
index 1dbfea842983..f8ef46a4dc37 100644
--- a/test/functional/test-animation.js
+++ b/test/functional/test-animation.js
@@ -51,10 +51,10 @@ describe('Animation', () => {
   });
 
   function runVsync() {
-    let tasks = vsyncTasks.slice(0);
+    const tasks = vsyncTasks.slice(0);
     vsyncTasks = [];
     tasks.forEach(function(task) {
-      let state = {};
+      const state = {};
       if (task.measure) {
         task.measure(state);
       }
@@ -68,7 +68,7 @@ describe('Animation', () => {
     anim.add(0, time => {tr1 = time;}, 0.8);
     anim.add(0.2, time => {tr2 = time;}, 0.8);
 
-    let ap = anim.start(1000);
+    const ap = anim.start(1000);
     let resolveCalled = false;
     ap.resolve_ = () => {
       resolveCalled = true;
@@ -140,10 +140,10 @@ describe('Animation', () => {
   it('should animate out-of-bounds time', () => {
     let tr1 = -1;
     // Linear curve between -0.5 and 1.5
-    let curve = time => {return time * 2 - 0.5;};
+    const curve = time => {return time * 2 - 0.5;};
     anim.add(0, time => {tr1 = time;}, 1, curve);
 
-    let ap = anim.start(1000);
+    const ap = anim.start(1000);
     let resolveCalled = false;
     ap.resolve_ = () => {
       resolveCalled = true;
@@ -174,7 +174,7 @@ describe('Animation', () => {
     anim.add(0, time => {tr1 = time;}, 0.8);
     anim.add(0.2, time => {tr2 = time;}, 0.8);
 
-    let ap = anim.start(1000);
+    const ap = anim.start(1000);
     let rejectCalled = false;
     ap.reject_ = () => {
       rejectCalled = true;
@@ -200,7 +200,7 @@ describe('Animation', () => {
     anim.add(0, time => {tr1 = time;}, 0.8);
     anim.add(0.2, time => {tr2 = time;}, 0.8);
 
-    let ap = anim.start(1000);
+    const ap = anim.start(1000);
     let rejectCalled = false;
     ap.reject_ = () => {
       rejectCalled = true;
@@ -226,7 +226,7 @@ describe('Animation', () => {
     anim.add(0, time => {tr1 = time;}, 0.8);
     anim.add(0.2, time => {tr2 = time;}, 0.8);
 
-    let ap = anim.start(1000);
+    const ap = anim.start(1000);
     let rejectCalled = false;
     ap.reject_ = () => {
       rejectCalled = true;
diff --git a/test/functional/test-base-element.js b/test/functional/test-base-element.js
index 9e39aee0de6d..3a3855eda6ee 100644
--- a/test/functional/test-base-element.js
+++ b/test/functional/test-base-element.js
@@ -28,7 +28,7 @@ describe('BaseElement', () => {
   });
 
   it('propagateAttributes - niente', () => {
-    let target = document.createElement('div');
+    const target = document.createElement('div');
     expect(target.hasAttributes()).to.be.false;
 
     element.propagateAttributes(['data-test1'], target);
@@ -39,7 +39,7 @@ describe('BaseElement', () => {
   });
 
   it('propagateAttributes', () => {
-    let target = document.createElement('div');
+    const target = document.createElement('div');
     expect(target.hasAttributes()).to.be.false;
 
     div.setAttribute('data-test1', 'abc');
@@ -59,7 +59,7 @@ describe('BaseElement', () => {
   });
 
   it('should register action', () => {
-    let handler = () => {};
+    const handler = () => {};
     element.registerAction('method1', handler);
     expect(element.actionMap_['method1']).to.equal(handler);
   });
@@ -71,14 +71,14 @@ describe('BaseElement', () => {
   });
 
   it('should execute registered action', () => {
-    let handler = sinon.spy();
+    const handler = sinon.spy();
     element.registerAction('method1', handler);
     element.executeAction({method: 'method1'}, false);
     expect(handler.callCount).to.equal(1);
   });
 
   it('should execute "activate" action without registration', () => {
-    let handler = sinon.spy();
+    const handler = sinon.spy();
     element.activate = handler;
     element.executeAction({method: 'activate'}, false);
     expect(handler.callCount).to.equal(1);
diff --git a/test/functional/test-curve.js b/test/functional/test-curve.js
index 35b6d002c4dd..3d0ac1256700 100644
--- a/test/functional/test-curve.js
+++ b/test/functional/test-curve.js
@@ -47,7 +47,7 @@ describe('Curve', () => {
     expect(getCurve(undefined)).to.equal(null);
 
     // Function is passed through.
-    let func = () => {};
+    const func = () => {};
     expect(getCurve(func)).to.equal(func);
 
     // String is translated.
diff --git a/test/functional/test-custom-element.js b/test/functional/test-custom-element.js
index a16f7dc20877..eae76e2526a6 100644
--- a/test/functional/test-custom-element.js
+++ b/test/functional/test-custom-element.js
@@ -25,7 +25,7 @@ import * as sinon from 'sinon';
 
 describe('CustomElement', () => {
 
-  let resources = resourcesFor(window);
+  const resources = resourcesFor(window);
   let testElementCreatedCallback;
   let testElementFirstAttachedCallback;
   let testElementBuildCallback;
@@ -67,11 +67,11 @@ describe('CustomElement', () => {
     }
   }
 
-  let ElementClass = document.registerElement('amp-test', {
+  const ElementClass = document.registerElement('amp-test', {
     prototype: createAmpElementProto(window, 'amp-test', TestElement)
   });
 
-  let StubElementClass = document.registerElement('amp-stub', {
+  const StubElementClass = document.registerElement('amp-stub', {
     prototype: createAmpElementProto(window, 'amp-stub', ElementStub)
   });
 
@@ -104,7 +104,7 @@ describe('CustomElement', () => {
 
 
   it('Element - createdCallback', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     expect(element.classList.contains('-amp-element')).to.equal(true);
     expect(element.isBuilt()).to.equal(false);
     expect(element.classList.contains('-amp-notbuilt')).to.equal(true);
@@ -117,7 +117,7 @@ describe('CustomElement', () => {
   });
 
   it('StubElement - createdCallback', () => {
-    let element = new StubElementClass();
+    const element = new StubElementClass();
     expect(element.classList.contains('-amp-element')).to.equal(true);
     expect(element.isBuilt()).to.equal(false);
     expect(element.classList.contains('-amp-notbuilt')).to.equal(true);
@@ -131,7 +131,7 @@ describe('CustomElement', () => {
 
 
   it('Element - updateLayoutBox', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     expect(element.layoutWidth_).to.equal(-1);
     expect(element.implementation_.layoutWidth_).to.equal(-1);
 
@@ -141,7 +141,7 @@ describe('CustomElement', () => {
   });
 
   it('StubElement - upgrade', () => {
-    let element = new StubElementClass();
+    const element = new StubElementClass();
     expect(element.isUpgraded()).to.equal(false);
     expect(testElementCreatedCallback.callCount).to.equal(0);
 
@@ -162,7 +162,7 @@ describe('CustomElement', () => {
 
 
   it('Element - build allowed', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     expect(element.classList.contains('-amp-element')).to.equal(true);
     expect(element.isBuilt()).to.equal(false);
     expect(element.classList.contains('-amp-notbuilt')).to.equal(true);
@@ -179,7 +179,7 @@ describe('CustomElement', () => {
   });
 
   it('Element - buildCallback cannot be called twice', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     expect(element.isBuilt()).to.equal(false);
     expect(testElementBuildCallback.callCount).to.equal(0);
 
@@ -197,7 +197,7 @@ describe('CustomElement', () => {
   });
 
   it('Element - build not allowed', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     expect(element.classList.contains('-amp-element')).to.equal(true);
     expect(element.isBuilt()).to.equal(false);
     expect(element.classList.contains('-amp-notbuilt')).to.equal(true);
@@ -214,7 +214,7 @@ describe('CustomElement', () => {
   });
 
   it('Element - build not allowed but forced', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     expect(element.classList.contains('-amp-element')).to.equal(true);
     expect(element.isBuilt()).to.equal(false);
     expect(element.classList.contains('-amp-notbuilt')).to.equal(true);
@@ -231,7 +231,7 @@ describe('CustomElement', () => {
   });
 
   it('StubElement - build never allowed', () => {
-    let element = new StubElementClass();
+    const element = new StubElementClass();
     expect(element.classList.contains('-amp-element')).to.equal(true);
     expect(element.isBuilt()).to.equal(false);
     expect(element.classList.contains('-amp-notbuilt')).to.equal(true);
@@ -248,7 +248,7 @@ describe('CustomElement', () => {
 
 
   it('Element - attachedCallback', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     element.setAttribute('layout', 'fill');
     expect(testElementFirstAttachedCallback.callCount).to.equal(0);
     expect(element.everAttached).to.equal(false);
@@ -264,7 +264,7 @@ describe('CustomElement', () => {
   });
 
   it('StubElement - attachedCallback', () => {
-    let element = new StubElementClass();
+    const element = new StubElementClass();
     element.setAttribute('layout', 'fill');
     expect(testElementFirstAttachedCallback.callCount).to.equal(0);
     expect(element.everAttached).to.equal(false);
@@ -292,7 +292,7 @@ describe('CustomElement', () => {
   });
 
   it('Element - detachedCallback', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     element.setAttribute('layout', 'fill');
     expect(testElementFirstAttachedCallback.callCount).to.equal(0);
     expect(element.everAttached).to.equal(false);
@@ -312,7 +312,7 @@ describe('CustomElement', () => {
 
 
   it('Element - layoutCallback before build', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     element.setAttribute('layout', 'fill');
     expect(testElementLayoutCallback.callCount).to.equal(0);
 
@@ -325,7 +325,7 @@ describe('CustomElement', () => {
   });
 
   it('StubElement - layoutCallback before build or upgrade', () => {
-    let element = new StubElementClass();
+    const element = new StubElementClass();
     element.setAttribute('layout', 'fill');
     expect(testElementLayoutCallback.callCount).to.equal(0);
 
@@ -348,13 +348,13 @@ describe('CustomElement', () => {
   });
 
   it('Element - layoutCallback', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     element.setAttribute('layout', 'fill');
     element.build(true);
     expect(element.isBuilt()).to.equal(true);
     expect(testElementLayoutCallback.callCount).to.equal(0);
 
-    let p = element.layoutCallback();
+    const p = element.layoutCallback();
     expect(testElementLayoutCallback.callCount).to.equal(1);
     return p.then(() => {
       expect(element.readyState).to.equal('complete');
@@ -363,18 +363,18 @@ describe('CustomElement', () => {
 
   it('Element - layoutCallback should call firstLayoutCompleted only once',
       () => {
-        let element = new ElementClass();
+        const element = new ElementClass();
         element.setAttribute('layout', 'fill');
         element.build(true);
 
-        let p = element.layoutCallback();
+        const p = element.layoutCallback();
         expect(testElementLayoutCallback.callCount).to.equal(1);
         expect(testElementFirstLayoutCompleted.callCount).to.equal(0);
         return p.then(() => {
           expect(testElementFirstLayoutCompleted.callCount).to.equal(1);
 
       // But not second time.
-          let p2 = element.layoutCallback();
+          const p2 = element.layoutCallback();
           expect(testElementLayoutCallback.callCount).to.equal(2);
           expect(testElementFirstLayoutCompleted.callCount).to.equal(1);
           return p2.then(() => {
@@ -384,7 +384,7 @@ describe('CustomElement', () => {
       });
 
   it('StubElement - layoutCallback', () => {
-    let element = new StubElementClass();
+    const element = new StubElementClass();
     element.setAttribute('layout', 'fill');
     resourcesMock.expects('upgraded').withExactArgs(element).once();
     element.upgrade(TestElement);
@@ -393,7 +393,7 @@ describe('CustomElement', () => {
     expect(element.isBuilt()).to.equal(true);
     expect(testElementLayoutCallback.callCount).to.equal(0);
 
-    let p = element.layoutCallback();
+    const p = element.layoutCallback();
     expect(testElementLayoutCallback.callCount).to.equal(1);
     return p.then(() => {
       expect(element.readyState).to.equal('complete');
@@ -402,12 +402,12 @@ describe('CustomElement', () => {
 
 
   it('should enqueue actions until built', () => {
-    let element = new ElementClass();
-    let handler = sinon.spy();
+    const element = new ElementClass();
+    const handler = sinon.spy();
     element.implementation_.executeAction = handler;
     expect(element.actionQueue_).to.not.equal(null);
 
-    let inv = {};
+    const inv = {};
     element.enqueAction(inv);
     expect(element.actionQueue_.length).to.equal(1);
     expect(element.actionQueue_[0]).to.equal(inv);
@@ -415,12 +415,12 @@ describe('CustomElement', () => {
   });
 
   it('should execute action immediately after built', () => {
-    let element = new ElementClass();
-    let handler = sinon.spy();
+    const element = new ElementClass();
+    const handler = sinon.spy();
     element.implementation_.executeAction = handler;
     element.build(true);
 
-    let inv = {};
+    const inv = {};
     element.enqueAction(inv);
     expect(handler.callCount).to.equal(1);
     expect(handler.getCall(0).args[0]).to.equal(inv);
@@ -428,12 +428,12 @@ describe('CustomElement', () => {
   });
 
   it('should dequeue all actions after build', () => {
-    let element = new ElementClass();
-    let handler = sinon.spy();
+    const element = new ElementClass();
+    const handler = sinon.spy();
     element.implementation_.executeAction = handler;
 
-    let inv1 = {};
-    let inv2 = {};
+    const inv1 = {};
+    const inv2 = {};
     element.enqueAction(inv1);
     element.enqueAction(inv2);
     expect(element.actionQueue_.length).to.equal(2);
@@ -453,37 +453,37 @@ describe('CustomElement', () => {
 
 
   it('should apply media condition', () => {
-    let element1 = new ElementClass();
+    const element1 = new ElementClass();
     element1.setAttribute('media', '(min-width: 1px)');
     element1.applySizesAndMediaQuery();
     expect(element1).to.not.have.class('-amp-hidden-by-media-query');
 
-    let element2 = new ElementClass();
+    const element2 = new ElementClass();
     element2.setAttribute('media', '(min-width: 1111111px)');
     element2.applySizesAndMediaQuery();
     expect(element2).to.have.class('-amp-hidden-by-media-query');
   });
 
   it('should apply sizes condition', () => {
-    let element1 = new ElementClass();
+    const element1 = new ElementClass();
     element1.setAttribute('sizes', '(min-width: 1px) 200px, 50vw');
     element1.applySizesAndMediaQuery();
     expect(element1.style.width).to.equal('200px');
 
-    let element2 = new ElementClass();
+    const element2 = new ElementClass();
     element2.setAttribute('sizes', '(min-width: 1111111px) 200px, 50vw');
     element2.applySizesAndMediaQuery();
     expect(element2.style.width).to.equal('50vw');
   });
 
   it('should change height without sizer', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     element.changeHeight(111);
     expect(element.style.height).to.equal('111px');
   });
 
   it('should change height with sizer', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
     element.sizerElement_ = document.createElement('div');
     element.changeHeight(111);
     expect(parseInt(element.sizerElement_.style.paddingTop, 10)).to.equal(0);
@@ -492,7 +492,7 @@ describe('CustomElement', () => {
 
 
   it('Element - documentInactiveCallback', () => {
-    let element = new ElementClass();
+    const element = new ElementClass();
 
     // Non-built element doesn't receive documentInactiveCallback.
     element.documentInactiveCallback();
@@ -505,7 +505,7 @@ describe('CustomElement', () => {
   });
 
   it('StubElement - documentInactiveCallback', () => {
-    let element = new StubElementClass();
+    const element = new StubElementClass();
 
     // Unupgraded document doesn't receive documentInactiveCallback.
     element.documentInactiveCallback();
@@ -514,7 +514,7 @@ describe('CustomElement', () => {
 
   describe('viewportCallback', () => {
     it('Element should allow, but not delegate before build', () => {
-      let element = new ElementClass();
+      const element = new ElementClass();
       element.setAttribute('layout', 'fill');
       expect(testElementViewportCallback.callCount).to.equal(0);
 
@@ -525,7 +525,7 @@ describe('CustomElement', () => {
     });
 
     it('StubElement - should not delegate before build or upgrade', () => {
-      let element = new StubElementClass();
+      const element = new StubElementClass();
       element.setAttribute('layout', 'fill');
       expect(testElementViewportCallback.callCount).to.equal(0);
 
@@ -546,7 +546,7 @@ describe('CustomElement', () => {
     });
 
     it('Element - should be called once built', () => {
-      let element = new ElementClass();
+      const element = new ElementClass();
       element.setAttribute('layout', 'fill');
       element.build(true);
       expect(element.isBuilt()).to.equal(true);
@@ -558,7 +558,7 @@ describe('CustomElement', () => {
     });
 
     it('StubElement - should be called once upgraded', () => {
-      let element = new StubElementClass();
+      const element = new StubElementClass();
       element.setAttribute('layout', 'fill');
       resourcesMock.expects('upgraded').withExactArgs(element).once();
       element.upgrade(TestElement);
@@ -573,7 +573,7 @@ describe('CustomElement', () => {
     });
 
     it('Element - should be called on built if in viewport', () => {
-      let element = new ElementClass();
+      const element = new ElementClass();
       element.setAttribute('layout', 'fill');
       element.viewportCallback(true);
       expect(element.isInViewport_).to.equal(true);
@@ -589,7 +589,7 @@ describe('CustomElement', () => {
 
 describe('CustomElement Service Elements', () => {
 
-  let StubElementClass = document.registerElement('amp-stub2', {
+  const StubElementClass = document.registerElement('amp-stub2', {
     prototype: createAmpElementProto(window, 'amp-stub2', ElementStub)
   });
 
@@ -607,7 +607,7 @@ describe('CustomElement Service Elements', () => {
   });
 
   function createWithAttr(attr) {
-    let child = document.createElement('div');
+    const child = document.createElement('div');
     child.setAttribute(attr, '');
     return child;
   }
@@ -624,12 +624,12 @@ describe('CustomElement Service Elements', () => {
     element.appendChild(document.createTextNode('abc'));
     element.appendChild(document.createElement('content'));
 
-    let nodes = element.getRealChildNodes();
+    const nodes = element.getRealChildNodes();
     expect(nodes.length).to.equal(2);
     expect(nodes[0].textContent).to.equal('abc');
     expect(nodes[1].tagName.toLowerCase()).to.equal('content');
 
-    let elements = element.getRealChildren();
+    const elements = element.getRealChildren();
     expect(elements.length).to.equal(1);
     expect(elements[0].tagName.toLowerCase()).to.equal('content');
   });
@@ -640,8 +640,8 @@ describe('CustomElement Service Elements', () => {
   });
 
   it('getPlaceholder should return the first placeholder', () => {
-    let placeholder1 = element.appendChild(createWithAttr('placeholder'));
-    let placeholder2 = element.appendChild(createWithAttr('placeholder'));
+    const placeholder1 = element.appendChild(createWithAttr('placeholder'));
+    const placeholder2 = element.appendChild(createWithAttr('placeholder'));
     expect(element.getPlaceholder()).to.equal(placeholder1);
   });
 
@@ -651,8 +651,8 @@ describe('CustomElement Service Elements', () => {
   });
 
   it('togglePlaceholder should do hide placeholder when found', () => {
-    let placeholder1 = element.appendChild(createWithAttr('placeholder'));
-    let placeholder2 = element.appendChild(createWithAttr('placeholder'));
+    const placeholder1 = element.appendChild(createWithAttr('placeholder'));
+    const placeholder2 = element.appendChild(createWithAttr('placeholder'));
     element.togglePlaceholder(false);
     expect(placeholder1).to.have.class('amp-hidden');
     expect(placeholder2).to.not.have.class('amp-hidden');
@@ -675,11 +675,11 @@ describe('CustomElement Loading Indicator', () => {
 
   class TestElement extends BaseElement {
   }
-  let ElementClass = document.registerElement('amp-test2', {
+  const ElementClass = document.registerElement('amp-test2', {
     prototype: createAmpElementProto(window, 'amp-test2', TestElement)
   });
 
-  let resources = resourcesFor(window);
+  const resources = resourcesFor(window);
   let sandbox;
   let clock;
   let element;
@@ -778,8 +778,8 @@ describe('CustomElement Loading Indicator', () => {
 
   it('should turn on already created', () => {
     element.prepareLoading_();
-    let container = element.loadingContainer_;
-    let indicator = element.loadingElement_;
+    const container = element.loadingContainer_;
+    const indicator = element.loadingElement_;
     element.toggleLoading_(true);
     expect(vsyncTasks).to.have.length.of(1);
 
@@ -817,7 +817,7 @@ describe('CustomElement Loading Indicator', () => {
 
 
   it('should turn off when exits viewport', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
+    const toggle = sandbox.spy(element, 'toggleLoading_');
     element.viewportCallback(false);
     expect(toggle.callCount).to.equal(1);
     expect(toggle.firstCall.args[0]).to.equal(false);
@@ -825,14 +825,14 @@ describe('CustomElement Loading Indicator', () => {
   });
 
   it('should NOT turn off when exits viewport but already laid out', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
+    const toggle = sandbox.spy(element, 'toggleLoading_');
     element.layoutCount_ = 1;
     element.viewportCallback(false);
     expect(toggle.callCount).to.equal(0);
   });
 
   it('should turn on when enters viewport', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
+    const toggle = sandbox.spy(element, 'toggleLoading_');
     element.viewportCallback(true);
     clock.tick(1000);
     expect(toggle.callCount).to.equal(1);
@@ -840,7 +840,7 @@ describe('CustomElement Loading Indicator', () => {
   });
 
   it('should NOT turn on when enters viewport but already laid out', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
+    const toggle = sandbox.spy(element, 'toggleLoading_');
     element.layoutCount_ = 1;
     element.viewportCallback(true);
     clock.tick(1000);
@@ -849,7 +849,7 @@ describe('CustomElement Loading Indicator', () => {
 
 
   it('should start loading when measured if already in viewport', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
+    const toggle = sandbox.spy(element, 'toggleLoading_');
     element.isInViewport_ = true;
     element.updateLayoutBox({top: 0, width: 300});
     expect(toggle.callCount).to.equal(1);
@@ -857,7 +857,7 @@ describe('CustomElement Loading Indicator', () => {
   });
 
   it('should create loading when measured if in the top window', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
+    const toggle = sandbox.spy(element, 'toggleLoading_');
     element.updateLayoutBox({top: 0, width: 300});
     expect(toggle.callCount).to.equal(0);
     expect(vsyncTasks).to.have.length.of(1);
@@ -868,7 +868,7 @@ describe('CustomElement Loading Indicator', () => {
 
 
   it('should toggle loading off after layout complete', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
+    const toggle = sandbox.spy(element, 'toggleLoading_');
     element.build(true);
     return element.layoutCallback().then(() => {
       expect(toggle.callCount).to.equal(1);
@@ -880,8 +880,8 @@ describe('CustomElement Loading Indicator', () => {
   });
 
   it('should toggle loading off after layout failed', () => {
-    let toggle = sandbox.spy(element, 'toggleLoading_');
-    let implMock = sandbox.mock(element.implementation_);
+    const toggle = sandbox.spy(element, 'toggleLoading_');
+    const implMock = sandbox.mock(element.implementation_);
     implMock.expects('layoutCallback').returns(Promise.reject());
     element.build(true);
     return element.layoutCallback().then(() => {
diff --git a/test/functional/test-document-state.js b/test/functional/test-document-state.js
index 427dfa321cc1..cfd2dcb0ecfb 100644
--- a/test/functional/test-document-state.js
+++ b/test/functional/test-document-state.js
@@ -51,14 +51,14 @@ describe('documentReady', () => {
 
   it('should call callback immediately when ready', () => {
     testDoc.readyState = 'complete';
-    let callback = sinon.spy();
+    const callback = sinon.spy();
     onDocumentReady(testDoc, callback);
     expect(callback.callCount).to.equal(1);
   });
 
   it('should wait to call callback until ready', () => {
     testDoc.readyState = 'loading';
-    let callback = sinon.spy();
+    const callback = sinon.spy();
     onDocumentReady(testDoc, callback);
     expect(callback.callCount).to.equal(0);
     expect(eventListeners['readystatechange']).to.not.equal(undefined);
@@ -72,7 +72,7 @@ describe('documentReady', () => {
 
   it('should wait to call callback for several loading events', () => {
     testDoc.readyState = 'loading';
-    let callback = sinon.spy();
+    const callback = sinon.spy();
     onDocumentReady(testDoc, callback);
     expect(callback.callCount).to.equal(0);
     expect(eventListeners['readystatechange']).to.not.equal(undefined);
@@ -125,41 +125,41 @@ describe('DocumentState', () => {
   });
 
   it('resolve vendor-prefixed properties', () => {
-    let otherDoc = {
+    const otherDoc = {
       webkitHidden: false,
       webkitVisibilityState: 'visible',
       addEventListener: (eventType, handler) => {},
       removeEventListener: (eventType, handler) => {}
     };
-    let other = new DocumentState({document: otherDoc});
+    const other = new DocumentState({document: otherDoc});
     expect(other.hiddenProp_).to.equal('webkitHidden');
     expect(other.visibilityStateProp_).to.equal('webkitVisibilityState');
     expect(other.visibilityChangeEvent_).to.equal('webkitVisibilitychange');
   });
 
   it('resolve no properties', () => {
-    let otherDoc = {
+    const otherDoc = {
       addEventListener: (eventType, handler) => {},
       removeEventListener: (eventType, handler) => {}
     };
-    let other = new DocumentState({document: otherDoc});
+    const other = new DocumentState({document: otherDoc});
     expect(other.hiddenProp_).to.equal(null);
     expect(other.visibilityStateProp_).to.equal(null);
     expect(other.visibilityChangeEvent_).to.equal(null);
   });
 
   it('should default hidden and visibilityState if unknown', () => {
-    let otherDoc = {
+    const otherDoc = {
       addEventListener: (eventType, handler) => {},
       removeEventListener: (eventType, handler) => {}
     };
-    let other = new DocumentState({document: otherDoc});
+    const other = new DocumentState({document: otherDoc});
     expect(other.isHidden()).to.equal(false);
     expect(other.getVisibilityState()).to.equal('visible');
   });
 
   it('should fire visibility change', () => {
-    let callback = sinon.spy();
+    const callback = sinon.spy();
     docState.onVisibilityChanged(callback);
 
     expect(docState.isHidden()).to.equal(false);
diff --git a/test/functional/test-dom.js b/test/functional/test-dom.js
index bb42bb26aa3b..3f23f2a6e029 100644
--- a/test/functional/test-dom.js
+++ b/test/functional/test-dom.js
@@ -20,7 +20,7 @@ import * as dom from '../../src/dom';
 describe('DOM', () => {
 
   it('should remove all children', () => {
-    let element = document.createElement('div');
+    const element = document.createElement('div');
     element.appendChild(document.createElement('div'));
     element.appendChild(document.createTextNode('ABC'));
     expect(element.children.length).to.equal(1);
@@ -34,11 +34,11 @@ describe('DOM', () => {
   });
 
   it('should copy all children', () => {
-    let element = document.createElement('div');
+    const element = document.createElement('div');
     element.appendChild(document.createElement('div'));
     element.appendChild(document.createTextNode('ABC'));
 
-    let other = document.createElement('div');
+    const other = document.createElement('div');
     dom.copyChildren(element, other);
 
     expect(element.children.length).to.equal(1);
@@ -52,9 +52,9 @@ describe('DOM', () => {
   });
 
   it('closest should find itself', () => {
-    let element = document.createElement('div');
+    const element = document.createElement('div');
 
-    let child = document.createElement('div');
+    const child = document.createElement('div');
     element.appendChild(child);
 
     expect(dom.closest(child, () => true)).to.equal(child);
@@ -63,12 +63,12 @@ describe('DOM', () => {
   });
 
   it('closest should find first match', () => {
-    let parent = document.createElement('parent');
+    const parent = document.createElement('parent');
 
-    let element = document.createElement('element');
+    const element = document.createElement('element');
     parent.appendChild(element);
 
-    let child = document.createElement('child');
+    const child = document.createElement('child');
     element.appendChild(child);
 
     expect(dom.closest(child, e => e.tagName == 'CHILD')).to.equal(child);
@@ -82,12 +82,12 @@ describe('DOM', () => {
   });
 
   it('closest should find first match', () => {
-    let parent = document.createElement('parent');
+    const parent = document.createElement('parent');
 
-    let element1 = document.createElement('element');
+    const element1 = document.createElement('element');
     parent.appendChild(element1);
 
-    let element2 = document.createElement('element');
+    const element2 = document.createElement('element');
     parent.appendChild(element2);
 
     expect(dom.elementByTag(parent, 'element')).to.equal(element1);
@@ -96,12 +96,12 @@ describe('DOM', () => {
 
 
   it('childElement should find first match', () => {
-    let parent = document.createElement('parent');
+    const parent = document.createElement('parent');
 
-    let element1 = document.createElement('element1');
+    const element1 = document.createElement('element1');
     parent.appendChild(element1);
 
-    let element2 = document.createElement('element2');
+    const element2 = document.createElement('element2');
     parent.appendChild(element2);
 
     expect(dom.childElement(parent, e => true)).to.equal(element1);
@@ -114,14 +114,14 @@ describe('DOM', () => {
   });
 
   it('childElementByAttr should find first match', () => {
-    let parent = document.createElement('parent');
+    const parent = document.createElement('parent');
 
-    let element1 = document.createElement('element1');
+    const element1 = document.createElement('element1');
     element1.setAttribute('attr1', '1');
     element1.setAttribute('attr12', '1');
     parent.appendChild(element1);
 
-    let element2 = document.createElement('element2');
+    const element2 = document.createElement('element2');
     element2.setAttribute('attr2', '2');
     element2.setAttribute('attr12', '2');
     parent.appendChild(element2);
diff --git a/test/functional/test-event-helper.js b/test/functional/test-event-helper.js
index 21b3e304f069..dbabb3fd09d9 100644
--- a/test/functional/test-event-helper.js
+++ b/test/functional/test-event-helper.js
@@ -74,9 +74,9 @@ describe('EventHelper', () => {
   });
 
   it('listenOnce', () => {
-    let event = getEvent('load');
+    const event = getEvent('load');
     let c = 0;
-    let handler = e => {
+    const handler = e => {
       c++;
       expect(e).to.equal(event);
     };
@@ -95,13 +95,13 @@ describe('EventHelper', () => {
   });
 
   it('listenOnce - cancel', () => {
-    let event = getEvent('load');
+    const event = getEvent('load');
     let c = 0;
-    let handler = e => {
+    const handler = e => {
       c++;
       expect(e).to.equal(event);
     };
-    let unlisten = listenOnce(element, 'load', handler);
+    const unlisten = listenOnce(element, 'load', handler);
 
     // Not fired yet.
     expect(c).to.equal(0);
@@ -115,8 +115,8 @@ describe('EventHelper', () => {
   });
 
   it('listenOncePromise - load event', () => {
-    let event = getEvent('load');
-    let promise = listenOncePromise(element, 'load').then(result => {
+    const event = getEvent('load');
+    const promise = listenOncePromise(element, 'load').then(result => {
       expect(result).to.equal(event);
     });
     loadObservable.fire(event);
@@ -124,8 +124,8 @@ describe('EventHelper', () => {
   });
 
   it('listenOncePromise - with time limit', () => {
-    let event = getEvent('load');
-    let promise = expect(listenOncePromise(element, 'load', false, 100))
+    const event = getEvent('load');
+    const promise = expect(listenOncePromise(element, 'load', false, 100))
       .to.eventually.become(event);
     sandbox.clock.tick(99);
     loadObservable.fire(event);
@@ -133,7 +133,7 @@ describe('EventHelper', () => {
   });
 
   it('listenOncePromise - timeout', () => {
-    let promise = expect(listenOncePromise(element, 'load', false, 100))
+    const promise = expect(listenOncePromise(element, 'load', false, 100))
       .to.eventually.be.rejectedWith('timeout');
     sandbox.clock.tick(101);
     return promise;
@@ -166,7 +166,7 @@ describe('EventHelper', () => {
   });
 
   it('loadPromise - load event', () => {
-    let promise = loadPromise(element).then(result => {
+    const promise = loadPromise(element).then(result => {
       expect(result).to.equal(element);
     });
     loadObservable.fire(getEvent('load'));
@@ -174,7 +174,7 @@ describe('EventHelper', () => {
   });
 
   it('loadPromise - error event', () => {
-    let promise = loadPromise(element).then(result => {
+    const promise = loadPromise(element).then(result => {
       assert.fail('must never be here: ' + result);
     }).catch(reason => {
     });
diff --git a/test/functional/test-focus-history.js b/test/functional/test-focus-history.js
index 94f7900c8c34..e84d673883fb 100644
--- a/test/functional/test-focus-history.js
+++ b/test/functional/test-focus-history.js
@@ -65,8 +65,8 @@ describe('FocusHistory', () => {
   });
 
   it('should push focused elements with timestamp', () => {
-    let el1 = document.createElement('div');
-    let el2 = document.createElement('div');
+    const el1 = document.createElement('div');
+    const el2 = document.createElement('div');
     eventListeners['focus']({target: el1});
     clock.tick(100);
     eventListeners['focus']({target: el2});
@@ -81,7 +81,7 @@ describe('FocusHistory', () => {
     windowEventListeners['blur']({});
     expect(focusHistory.history_.length).to.equal(0);
 
-    let el1 = document.createElement('div');
+    const el1 = document.createElement('div');
     testDoc.activeElement = el1;
     clock.tick(1000);
     expect(focusHistory.history_.length).to.equal(1);
@@ -89,8 +89,8 @@ describe('FocusHistory', () => {
   });
 
   it('should push and purge', () => {
-    let el1 = document.createElement('div');
-    let el2 = document.createElement('div');
+    const el1 = document.createElement('div');
+    const el2 = document.createElement('div');
     eventListeners['focus']({target: el1});
     clock.tick(100000);
     eventListeners['focus']({target: el2});
@@ -101,8 +101,8 @@ describe('FocusHistory', () => {
   });
 
   it('should replace second push with a new timestamp', () => {
-    let el1 = document.createElement('div');
-    let el2 = document.createElement('div');
+    const el1 = document.createElement('div');
+    const el2 = document.createElement('div');
     eventListeners['focus']({target: el1});
     clock.tick(100);
     eventListeners['focus']({target: el2});
@@ -117,8 +117,8 @@ describe('FocusHistory', () => {
   });
 
   it('should purge elements before timestamp', () => {
-    let el1 = document.createElement('div');
-    let el2 = document.createElement('div');
+    const el1 = document.createElement('div');
+    const el2 = document.createElement('div');
     clock.tick(100);
     eventListeners['focus']({target: el1});  // time=100
     clock.tick(100);
@@ -139,7 +139,7 @@ describe('FocusHistory', () => {
   });
 
   it('should return false when nothing matches descendants', () => {
-    let el0 = document.createElement('div');
+    const el0 = document.createElement('div');
     expect(focusHistory.hasDescendantsOf(el0)).to.be.false;
 
     eventListeners['focus']({target: document.createElement('div')});
@@ -147,8 +147,8 @@ describe('FocusHistory', () => {
   });
 
   it('should check active element for descendants', () => {
-    let el0 = document.createElement('div');
-    let el01 = document.createElement('div');
+    const el0 = document.createElement('div');
+    const el01 = document.createElement('div');
     el0.appendChild(el01);
 
     testDoc.activeElement = el0;
@@ -161,8 +161,8 @@ describe('FocusHistory', () => {
   });
 
   it('should check history descendants', () => {
-    let el0 = document.createElement('div');
-    let el01 = document.createElement('div');
+    const el0 = document.createElement('div');
+    const el01 = document.createElement('div');
     el0.appendChild(el01);
     eventListeners['focus']({target: el01});
     expect(focusHistory.hasDescendantsOf(el0)).to.be.true;
diff --git a/test/functional/test-gesture-recognizers.js b/test/functional/test-gesture-recognizers.js
index 79d294098f55..7de136e43e6c 100644
--- a/test/functional/test-gesture-recognizers.js
+++ b/test/functional/test-gesture-recognizers.js
@@ -55,7 +55,7 @@ describe('TapRecognizer', () => {
 
 
   it('should allow single-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}]});
     expect(res).to.equal(true);
     expect(recognizer.startX_).to.equal(101);
@@ -63,7 +63,7 @@ describe('TapRecognizer', () => {
   });
 
   it('should deny two-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}, {}]});
     expect(res).to.equal(false);
     expect(recognizer.startX_).to.equal(0);
@@ -151,7 +151,7 @@ describe('DoubletapRecognizer', () => {
 
 
   it('should allow single-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}]});
     expect(res).to.equal(true);
     expect(recognizer.startX_).to.equal(101);
@@ -159,7 +159,7 @@ describe('DoubletapRecognizer', () => {
   });
 
   it('should deny two-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}, {}]});
     expect(res).to.equal(false);
     expect(recognizer.startX_).to.equal(0);
@@ -265,7 +265,7 @@ describe('SwipeXYRecognizer', () => {
 
 
   it('should allow single-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}]});
     expect(res).to.equal(true);
     expect(recognizer.startX_).to.equal(101);
@@ -273,7 +273,7 @@ describe('SwipeXYRecognizer', () => {
   });
 
   it('should deny two-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}, {}]});
     expect(res).to.equal(false);
     expect(recognizer.startX_).to.equal(0);
@@ -341,7 +341,7 @@ describe('SwipeXYRecognizer', () => {
   });
 
   it('should emit on touchmove after start', () => {
-    let event = {touches: [{clientX: 111, clientY: 211}]};
+    const event = {touches: [{clientX: 111, clientY: 211}]};
 
     recognizer.startX_ = recognizer.prevX_ = 101;
     recognizer.startY_ = recognizer.prevY_ = 201;
@@ -357,14 +357,14 @@ describe('SwipeXYRecognizer', () => {
     gesturesMock.expects('signalEnd_').never();
 
     clock.tick(10);
-    let res = recognizer.onTouchMove(event);
+    const res = recognizer.onTouchMove(event);
     expect(res).to.equal(true);
     expect(recognizer.lastX_).to.equal(111);
     expect(recognizer.lastY_).to.equal(211);
   });
 
   it('should stop on touchend; velocity doesn\'t change', () => {
-    let event = {};
+    const event = {};
 
     recognizer.startX_ = recognizer.prevX_ = 101;
     recognizer.startY_ = recognizer.prevY_ = 201;
@@ -389,7 +389,7 @@ describe('SwipeXYRecognizer', () => {
   });
 
   it('should stop on touchend; velocity changes', () => {
-    let event = {};
+    const event = {};
 
     recognizer.startX_ = 101;
     recognizer.startY_ = 201;
@@ -450,7 +450,7 @@ describe('TapzoomRecognizer', () => {
 
 
   it('should allow single-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}]});
     expect(res).to.equal(true);
     expect(recognizer.startX_).to.equal(101);
@@ -458,7 +458,7 @@ describe('TapzoomRecognizer', () => {
   });
 
   it('should deny two-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}, {}]});
     expect(res).to.equal(false);
     expect(recognizer.startX_).to.equal(0);
@@ -552,7 +552,7 @@ describe('TapzoomRecognizer', () => {
   });
 
   it('should emit on touchmove after start', () => {
-    let event = {touches: [{clientX: 111, clientY: 211}]};
+    const event = {touches: [{clientX: 111, clientY: 211}]};
 
     recognizer.startX_ = recognizer.prevX_ = 101;
     recognizer.startY_ = recognizer.prevY_ = 201;
@@ -567,14 +567,14 @@ describe('TapzoomRecognizer', () => {
     gesturesMock.expects('signalEnd_').never();
 
     clock.tick(10);
-    let res = recognizer.onTouchMove(event);
+    const res = recognizer.onTouchMove(event);
     expect(res).to.equal(true);
     expect(recognizer.lastX_).to.equal(111);
     expect(recognizer.lastY_).to.equal(211);
   });
 
   it('should stop on touchend', () => {
-    let event = {};
+    const event = {};
 
     recognizer.startX_ = recognizer.prevX_ = 101;
     recognizer.startY_ = recognizer.prevY_ = 201;
@@ -637,7 +637,7 @@ describe('PinchRecognizer', () => {
 
 
   it('should deny single-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 101, clientY: 201}]});
     expect(res).to.equal(false);
     expect(recognizer.startX1_).to.equal(0);
@@ -647,7 +647,7 @@ describe('PinchRecognizer', () => {
   });
 
   it('should allow two-point touchstart', () => {
-    let res = recognizer.onTouchStart({touches:
+    const res = recognizer.onTouchStart({touches:
         [{clientX: 90, clientY: 80},
          {clientX: 110, clientY: 120}]});
     expect(res).to.equal(true);
@@ -767,7 +767,7 @@ describe('PinchRecognizer', () => {
     clock.tick(10);
     recognizer.acceptStart();
 
-    let event = {touches:
+    const event = {touches:
         [{clientX: 70, clientY: 60},
          {clientX: 130, clientY: 140}]};
     gesturesMock.expects('signalEmit_').withExactArgs(recognizer,
@@ -784,7 +784,7 @@ describe('PinchRecognizer', () => {
     gesturesMock.expects('signalEnd_').never();
 
     clock.tick(10);
-    let res = recognizer.onTouchMove(event);
+    const res = recognizer.onTouchMove(event);
     expect(res).to.equal(true);
   });
 
@@ -801,7 +801,7 @@ describe('PinchRecognizer', () => {
     clock.tick(10);
     recognizer.acceptStart();
 
-    let event = {};
+    const event = {};
     gesturesMock.expects('signalEmit_').withExactArgs(recognizer,
         sinon.match(data => {
           return (data.first === false && data.last === true &&
@@ -831,7 +831,7 @@ describe('PinchRecognizer', () => {
     clock.tick(10);
     recognizer.acceptStart();
 
-    let event = {};
+    const event = {};
     gesturesMock.expects('signalEmit_').withExactArgs(recognizer,
         sinon.match(data => {
           return (data.first === false && data.last === true &&
diff --git a/test/functional/test-gesture.js b/test/functional/test-gesture.js
index 0bb81b23c423..dfa3f6e0fad3 100644
--- a/test/functional/test-gesture.js
+++ b/test/functional/test-gesture.js
@@ -79,7 +79,7 @@ describe('Gestures', () => {
 
 
   it('onPointerDown should be called', () => {
-    let handler = sinon.spy();
+    const handler = sinon.spy();
     gestures.onPointerDown(handler);
     sendEvent({type: 'touchstart'});
     expect(handler.callCount).to.equal(1);
@@ -223,13 +223,13 @@ describe('Gestures', () => {
   });
 
   it('should allow emit', () => {
-    let data = {};
-    let event = {};
+    const data = {};
+    const event = {};
     clock.tick(1);
     gestures.eventing_ = recognizer;
     gestures.signalEmit_(recognizer, data, event);
     expect(onGesture.callCount).to.equal(1);
-    let gesture = onGesture.getCall(0).args[0];
+    const gesture = onGesture.getCall(0).args[0];
     expect(gesture.type).to.equal('test');
     expect(gesture.data).to.equal(data);
     expect(gesture.event).to.equal(event);
@@ -267,7 +267,7 @@ describe('Gestures', () => {
 
   it('should allow youngest to start', () => {
     gestures.onGesture(Test2Recognizer, () => {});
-    let recognizer2Mock = sandbox.mock(gestures.recognizers_[1]);
+    const recognizer2Mock = sandbox.mock(gestures.recognizers_[1]);
 
     gestures.ready_[0] = 10;
     gestures.ready_[1] = 9;
@@ -281,7 +281,7 @@ describe('Gestures', () => {
 
 
   it('should allow event to propagate when nothing happening', () => {
-    let event = {
+    const event = {
       type: 'touchend',
       preventDefault: sinon.spy(),
       stopPropagation: sinon.spy(),
@@ -293,7 +293,7 @@ describe('Gestures', () => {
 
   it('should cancel event when eventing', () => {
     gestures.eventing_ = recognizer;
-    let event = {
+    const event = {
       type: 'touchend',
       preventDefault: sinon.spy(),
       stopPropagation: sinon.spy(),
@@ -309,7 +309,7 @@ describe('Gestures', () => {
     expect(gestures.eventing_).to.equal(null);
     expect(gestures.wasEventing_).to.equal(true);
 
-    let event = {
+    const event = {
       type: 'touchend',
       preventDefault: sinon.spy(),
       stopPropagation: sinon.spy(),
@@ -322,7 +322,7 @@ describe('Gestures', () => {
 
   it('should cancel event when anyone is ready', () => {
     gestures.ready_[0] = 1;
-    let event = {
+    const event = {
       type: 'touchend',
       preventDefault: sinon.spy(),
       stopPropagation: sinon.spy(),
diff --git a/test/functional/test-history.js b/test/functional/test-history.js
index 1f1a778c14df..c49c17a612f7 100644
--- a/test/functional/test-history.js
+++ b/test/functional/test-history.js
@@ -32,7 +32,7 @@ describe('History', () => {
     sandbox = sinon.sandbox.create();
     clock = sandbox.useFakeTimers();
 
-    let binding = {
+    const binding = {
       cleanup_: () => {},
       setOnStackIndexUpdated: callback => {
         onStackIndexUpdated = callback;
@@ -63,7 +63,7 @@ describe('History', () => {
   });
 
   it('should push new state', () => {
-    let onPop = sinon.spy();
+    const onPop = sinon.spy();
     bindingMock.expects('push').withExactArgs()
         .returns(Promise.resolve(11)).once();
     return history.push(onPop).then(historyId => {
@@ -75,7 +75,7 @@ describe('History', () => {
   });
 
   it('should pop previously pushed state', () => {
-    let onPop = sinon.spy();
+    const onPop = sinon.spy();
     bindingMock.expects('push').withExactArgs()
         .returns(Promise.resolve(11)).once();
     bindingMock.expects('pop').withExactArgs(11)
@@ -95,7 +95,7 @@ describe('History', () => {
   });
 
   it('should return and call callback when history popped', () => {
-    let onPop = sinon.spy();
+    const onPop = sinon.spy();
     bindingMock.expects('push').withExactArgs()
         .returns(Promise.resolve(11)).once();
     return history.push(onPop).then(historyId => {
@@ -147,7 +147,7 @@ describe('HistoryBindingNatural', () => {
     history.origPushState_({'AMP.History': window.history.length}, undefined);
     history.origReplaceState_({'AMP.History': window.history.length - 2},
         undefined);
-    let history2 = new HistoryBindingNatural_(window);
+    const history2 = new HistoryBindingNatural_(window);
     expect(history2.stackIndex_).to.equal(window.history.length - 2);
     expect(history2.startIndex_).to.equal(window.history.length - 2);
     expect(history.unsupportedState_['AMP.History']).to.equal(
@@ -180,12 +180,12 @@ describe('HistoryBindingNatural', () => {
   it('should not pass in `url` argument to original replace state if ' +
     'parameter is undefined', () => {
     let argumentLength = 0;
-    let origReplace = window.history.replaceState;
+    const origReplace = window.history.replaceState;
     window.history.replaceState = function() {
       argumentLength = arguments.length;
     };
 
-    let history2 = new HistoryBindingNatural_(window);
+    const history2 = new HistoryBindingNatural_(window);
 
     expect(argumentLength).to.equal(2);
 
@@ -209,10 +209,10 @@ describe('HistoryBindingNatural', () => {
       expect(onStackIndexUpdated.callCount).to.equal(1);
       expect(onStackIndexUpdated.getCall(0).args[0]).to.equal(
           window.history.length - 1);
-      let histPromise = listenOncePromise(window, 'popstate').then(() => {
+      const histPromise = listenOncePromise(window, 'popstate').then(() => {
         clock.tick(100);
       });
-      let popPromise = history.pop(stackIndex);
+      const popPromise = history.pop(stackIndex);
       return histPromise.then(hist => {
         return popPromise.then(pop => {
           expect(pop).to.equal(window.history.length - 2);
@@ -232,7 +232,7 @@ describe('HistoryBindingNatural', () => {
       expect(onStackIndexUpdated.callCount).to.equal(1);
       expect(onStackIndexUpdated.getCall(0).args[0]).to.equal(
           window.history.length - 1);
-      let histPromise = listenOncePromise(window, 'popstate').then(() => {
+      const histPromise = listenOncePromise(window, 'popstate').then(() => {
         clock.tick(100);
       });
       window.history.go(-1);
@@ -264,7 +264,7 @@ describe('HistoryBindingVirtual', () => {
     clock = sandbox.useFakeTimers();
     onStackIndexUpdated = sinon.spy();
     viewerHistoryPoppedHandler = undefined;
-    let viewer = {
+    const viewer = {
       onHistoryPoppedEvent: handler => {
         viewerHistoryPoppedHandler = handler;
         return () => {};
diff --git a/test/functional/test-input.js b/test/functional/test-input.js
index 781a94b28fcd..71c734b7ab7d 100644
--- a/test/functional/test-input.js
+++ b/test/functional/test-input.js
@@ -106,7 +106,7 @@ describe('Input', () => {
     });
     expect(mouseDetected).to.equal(undefined);
 
-    let p = input.onMouseMove_();
+    const p = input.onMouseMove_();
     expect(eventListeners['click']).to.not.equal(undefined);
     clock.tick(350);
 
@@ -126,7 +126,7 @@ describe('Input', () => {
     });
     expect(mouseDetected).to.equal(undefined);
 
-    let p = input.onMouseMove_();
+    const p = input.onMouseMove_();
     eventListeners['click']();
 
     return p.then(() => {
@@ -146,7 +146,7 @@ describe('Input', () => {
     });
     expect(mouseDetected).to.equal(undefined);
 
-    let p = input.onMouseMove_();
+    const p = input.onMouseMove_();
     input.mouseConfirmAttemptCount_ = 100;
     eventListeners['mousemove'] = undefined;
     eventListeners['click']();
diff --git a/test/functional/test-layout-rect.js b/test/functional/test-layout-rect.js
index 4215c4202a03..537fcc0e38be 100644
--- a/test/functional/test-layout-rect.js
+++ b/test/functional/test-layout-rect.js
@@ -19,7 +19,7 @@ import * as lr from '../../src/layout-rect';
 describe('LayoutRect', () => {
 
   it('layoutRectLtwh', () => {
-    let rect = lr.layoutRectLtwh(1, 2, 3, 4);
+    const rect = lr.layoutRectLtwh(1, 2, 3, 4);
     expect(rect.left).to.equal(1);
     expect(rect.top).to.equal(2);
     expect(rect.width).to.equal(3);
@@ -29,17 +29,17 @@ describe('LayoutRect', () => {
   });
 
   it('layoutRectsOverlap', () => {
-    let rect1 = lr.layoutRectLtwh(10, 20, 30, 40);
-    let rect2 = lr.layoutRectLtwh(40, 60, 10, 10);
-    let rect3 = lr.layoutRectLtwh(41, 60, 10, 10);
+    const rect1 = lr.layoutRectLtwh(10, 20, 30, 40);
+    const rect2 = lr.layoutRectLtwh(40, 60, 10, 10);
+    const rect3 = lr.layoutRectLtwh(41, 60, 10, 10);
     expect(lr.layoutRectsOverlap(rect1, rect2)).to.equal(true);
     expect(lr.layoutRectsOverlap(rect1, rect3)).to.equal(false);
     expect(lr.layoutRectsOverlap(rect2, rect3)).to.equal(true);
   });
 
   it('expandLayoutRect', () => {
-    let rect1 = lr.layoutRectLtwh(10, 20, 30, 40);
-    let rect2 = lr.expandLayoutRect(rect1, 2, 3);
+    const rect1 = lr.layoutRectLtwh(10, 20, 30, 40);
+    const rect2 = lr.expandLayoutRect(rect1, 2, 3);
     expect(rect2.left).to.equal(10 - 30 * 2);
     expect(rect2.right).to.equal(40 + 30 * 2);
     expect(rect2.width).to.equal(30 + 30 * 4);
@@ -49,8 +49,8 @@ describe('LayoutRect', () => {
   });
 
   it('moveLayoutRect', () => {
-    let rect1 = lr.layoutRectLtwh(10, 20, 30, 40);
-    let rect2 = lr.moveLayoutRect(rect1, 2, 3);
+    const rect1 = lr.layoutRectLtwh(10, 20, 30, 40);
+    const rect2 = lr.moveLayoutRect(rect1, 2, 3);
     expect(rect2.left).to.equal(rect1.left + 2);
     expect(rect2.right).to.equal(rect1.right + 2);
     expect(rect2.width).to.equal(rect1.width);
@@ -60,7 +60,7 @@ describe('LayoutRect', () => {
   });
 
   it('layoutRectFromDomRect', () => {
-    let rect = lr.layoutRectFromDomRect({top: 11, left: 12, width: 111,
+    const rect = lr.layoutRectFromDomRect({top: 11, left: 12, width: 111,
         height: 222});
     expect(rect.top).to.equal(11);
     expect(rect.left).to.equal(12);
diff --git a/test/functional/test-layout.js b/test/functional/test-layout.js
index 34cb0dcc5597..5ffbf9523e15 100644
--- a/test/functional/test-layout.js
+++ b/test/functional/test-layout.js
@@ -257,14 +257,14 @@ describe('Layout', () => {
 
 
   it('should configure natural dimensions; default layout', () => {
-    let pixel = document.createElement('amp-pixel');
+    const pixel = document.createElement('amp-pixel');
     expect(applyLayout_(pixel)).to.equal(Layout.FIXED);
     expect(pixel.style.width).to.equal('1px');
     expect(pixel.style.height).to.equal('1px');
   });
 
   it('should configure natural dimensions; default layout; with width', () => {
-    let pixel = document.createElement('amp-pixel');
+    const pixel = document.createElement('amp-pixel');
     pixel.setAttribute('width', '11');
     expect(applyLayout_(pixel)).to.equal(Layout.FIXED);
     expect(pixel.style.width).to.equal('11px');
@@ -272,7 +272,7 @@ describe('Layout', () => {
   });
 
   it('should configure natural dimensions; default layout; with height', () => {
-    let pixel = document.createElement('amp-pixel');
+    const pixel = document.createElement('amp-pixel');
     pixel.setAttribute('height', '11');
     expect(applyLayout_(pixel)).to.equal(Layout.FIXED);
     expect(pixel.style.width).to.equal('1px');
@@ -280,7 +280,7 @@ describe('Layout', () => {
   });
 
   it('should configure natural dimensions; layout=fixed', () => {
-    let pixel = document.createElement('amp-pixel');
+    const pixel = document.createElement('amp-pixel');
     pixel.setAttribute('layout', 'fixed');
     expect(applyLayout_(pixel)).to.equal(Layout.FIXED);
     expect(pixel.style.width).to.equal('1px');
@@ -288,7 +288,7 @@ describe('Layout', () => {
   });
 
   it('should configure natural dimensions; layout=fixed-height', () => {
-    let pixel = document.createElement('amp-pixel');
+    const pixel = document.createElement('amp-pixel');
     pixel.setAttribute('layout', 'fixed-height');
     expect(applyLayout_(pixel)).to.equal(Layout.FIXED_HEIGHT);
     expect(pixel.style.height).to.equal('1px');
diff --git a/test/functional/test-motion.js b/test/functional/test-motion.js
index 942617f24dc2..255d25d6e225 100644
--- a/test/functional/test-motion.js
+++ b/test/functional/test-motion.js
@@ -87,14 +87,16 @@ describe('Motion continueMotion', () => {
   function testContinuation(maxVelocity, haltAfterTime) {
     let resultX = null;
     let resultY = null;
-    let motion = continueMotion(141, 104, maxVelocity, maxVelocity, (x, y) => {
-      resultX = x;
-      resultY = y;
-      return true;
-    }, vsync);
+    const motion = continueMotion(141, 104, maxVelocity, maxVelocity,
+        (x, y) => {
+          resultX = x;
+          resultY = y;
+          return true;
+        },
+        vsync);
 
     expect(vsyncTasks.length).to.equal(1);
-    let mutator = vsyncTasks[0];
+    const mutator = vsyncTasks[0];
     vsyncTasks = [];
 
     resultX = resultY = null;
@@ -102,13 +104,13 @@ describe('Motion continueMotion', () => {
     expect(resultX).to.be.closeTo(141, 1e-3);
     expect(resultY).to.be.closeTo(104, 1e-3);
 
-    let values = [];
+    const values = [];
     let time = 0;
     let continuing = true;
     do {
       resultX = resultY = null;
       clock.tick(100);
-      let prev = time;
+      const prev = time;
       time += 100;
       continuing = mutator(time, time - prev, {});
       if (resultX != null) {
@@ -124,7 +126,7 @@ describe('Motion continueMotion', () => {
   }
 
   it('should follow positive inertia', () => {
-    let values = testContinuation(0.665, 0);
+    const values = testContinuation(0.665, 0);
     expect(values.length).to.equal(12);
     expect(values[0]).to.be.closeTo(66, 1);
     expect(values[1]).to.be.closeTo(115, 1);
@@ -134,7 +136,7 @@ describe('Motion continueMotion', () => {
   });
 
   it('should halt when requested while following positive inertia', () => {
-    let values = testContinuation(0.665, 300);
+    const values = testContinuation(0.665, 300);
     expect(values.length).to.equal(3);
     expect(values[0]).to.be.closeTo(66, 1);
     expect(values[1]).to.be.closeTo(115, 1);
@@ -142,7 +144,7 @@ describe('Motion continueMotion', () => {
   });
 
   it('should follow negative inertia', () => {
-    let values = testContinuation(-0.665, 0);
+    const values = testContinuation(-0.665, 0);
     expect(values.length).to.equal(12);
     expect(values[0]).to.be.closeTo(-66, 1);
     expect(values[1]).to.be.closeTo(-115, 1);
@@ -152,7 +154,7 @@ describe('Motion continueMotion', () => {
   });
 
   it('should halt when requested while following negative inertia', () => {
-    let values = testContinuation(-0.665, 300);
+    const values = testContinuation(-0.665, 300);
     expect(values.length).to.equal(3);
     expect(values[0]).to.be.closeTo(-66, 1);
     expect(values[1]).to.be.closeTo(-115, 1);
diff --git a/test/functional/test-observable.js b/test/functional/test-observable.js
index aa736abbb45c..f0811cb4a00a 100644
--- a/test/functional/test-observable.js
+++ b/test/functional/test-observable.js
@@ -34,16 +34,16 @@ describe('Observable', () => {
 
   it('add-remove-fire', () => {
     let observer1Called = 0;
-    let observer1 = () => {
+    const observer1 = () => {
       observer1Called++;
     };
-    let observer1Key = observable.add(observer1);
+    const observer1Key = observable.add(observer1);
 
     let observer2Called = 0;
-    let observer2 = () => {
+    const observer2 = () => {
       observer2Called++;
     };
-    let observer2Key = observable.add(observer2);
+    const observer2Key = observable.add(observer2);
 
     expect(observer1Called).to.equal(0);
     expect(observer2Called).to.equal(0);
diff --git a/test/functional/test-pass.js b/test/functional/test-pass.js
index 95966b8f0838..7fff2ce68610 100644
--- a/test/functional/test-pass.js
+++ b/test/functional/test-pass.js
@@ -80,7 +80,7 @@ describe('Pass', () => {
     timerMock.expects('cancel').never();
     pass.schedule(111);
     // Will never schedule b/c there's an earlier pass still pending.
-    let isScheduled = pass.schedule(222);
+    const isScheduled = pass.schedule(222);
     expect(isScheduled).to.equal(false);
   });
 
@@ -92,7 +92,7 @@ describe('Pass', () => {
     timerMock.expects('cancel').withExactArgs(1).once();
     pass.schedule(222);
     // Will re-schedule b/c the existing pass is later.
-    let isScheduled = pass.schedule(111);
+    const isScheduled = pass.schedule(111);
     expect(isScheduled).to.equal(true);
   });
 
diff --git a/test/functional/test-platform.js b/test/functional/test-platform.js
index dc33e324fbea..f44c047fbcb0 100644
--- a/test/functional/test-platform.js
+++ b/test/functional/test-platform.js
@@ -28,7 +28,7 @@ describe('Platform', () => {
   });
 
   function testUserAgent(userAgentString) {
-    let platform = new Platform({navigator: {userAgent: userAgentString}});
+    const platform = new Platform({navigator: {userAgent: userAgentString}});
     expect(platform.isIos()).to.equal(isIos);
     expect(platform.isChrome()).to.equal(isChrome);
     expect(platform.isSafari()).to.equal(isSafari);
diff --git a/test/functional/test-pull-to-refresh.js b/test/functional/test-pull-to-refresh.js
index b85201dfd37f..7fafc91ca9d8 100644
--- a/test/functional/test-pull-to-refresh.js
+++ b/test/functional/test-pull-to-refresh.js
@@ -29,7 +29,7 @@ describe('PullToRefreshBlocker', () => {
     sandbox = sinon.sandbox.create();
 
     eventListeners = {};
-    let documentApi = {
+    const documentApi = {
       addEventListener: (eventType, handler) => {
         eventListeners[eventType] = handler;
       },
@@ -40,7 +40,7 @@ describe('PullToRefreshBlocker', () => {
       }
     };
 
-    let viewportApi = {
+    const viewportApi = {
       getTop: () => 0
     };
     viewportMock = sandbox.mock(viewportApi);
@@ -134,7 +134,7 @@ describe('PullToRefreshBlocker', () => {
     sendEvent({type: 'touchstart', touches: [{clientY: 111}]});
     expect(blocker.tracking_).to.equal(true);
 
-    let preventDefault = sinon.spy();
+    const preventDefault = sinon.spy();
     sendEvent({type: 'touchmove', touches: [{clientY: 112}]},
         preventDefault);
     expect(blocker.tracking_).to.equal(false);
@@ -150,7 +150,7 @@ describe('PullToRefreshBlocker', () => {
     sendEvent({type: 'touchstart', touches: [{clientY: 111}]});
     expect(blocker.tracking_).to.equal(true);
 
-    let preventDefault = sinon.spy();
+    const preventDefault = sinon.spy();
     sendEvent({type: 'touchmove', touches: [{clientY: 100}]},
         preventDefault);
     expect(blocker.tracking_).to.equal(false);
@@ -166,7 +166,7 @@ describe('PullToRefreshBlocker', () => {
     sendEvent({type: 'touchstart', touches: [{clientY: 111}]});
     expect(blocker.tracking_).to.equal(true);
 
-    let preventDefault = sinon.spy();
+    const preventDefault = sinon.spy();
     sendEvent({type: 'touchmove', touches: [{clientY: 111}]},
         preventDefault);
     expect(blocker.tracking_).to.equal(true);
diff --git a/test/functional/test-resources.js b/test/functional/test-resources.js
index c6053acf593b..bcc8f5349226 100644
--- a/test/functional/test-resources.js
+++ b/test/functional/test-resources.js
@@ -41,9 +41,9 @@ describe('Resources', () => {
   });
 
   it('should calculate correct calcTaskScore', () => {
-    let viewportRect = layoutRectLtwh(0, 100, 300, 400);
+    const viewportRect = layoutRectLtwh(0, 100, 300, 400);
     // Task 1 is right in the middle of the viewport and priority 0
-    let task_vp0_p0 = {
+    const task_vp0_p0 = {
       resource: {
         getLayoutBox() {
           return layoutRectLtwh(0, 100, 300, 100);
@@ -52,7 +52,7 @@ describe('Resources', () => {
       priority: 0
     };
     // Task 2 is in the viewport and priority 1
-    let task_vp0_p1 = {
+    const task_vp0_p1 = {
       resource: {
         getLayoutBox() {
           return layoutRectLtwh(0, 100, 300, 100);
@@ -61,7 +61,7 @@ describe('Resources', () => {
       priority: 1
     };
     // Task 3 is above viewport and priority 0
-    let task_vpu_p0 = {
+    const task_vpu_p0 = {
       resource: {
         getLayoutBox() {
           return layoutRectLtwh(0, 0, 300, 50);
@@ -70,7 +70,7 @@ describe('Resources', () => {
       priority: 0
     };
     // Task 4 is above viewport and priority 0
-    let task_vpu_p1 = {
+    const task_vpu_p1 = {
       resource: {
         getLayoutBox() {
           return layoutRectLtwh(0, 0, 300, 50);
@@ -79,7 +79,7 @@ describe('Resources', () => {
       priority: 1
     };
     // Task 5 is below viewport and priority 0
-    let task_vpd_p0 = {
+    const task_vpd_p0 = {
       resource: {
         getLayoutBox() {
           return layoutRectLtwh(0, 600, 300, 50);
@@ -88,7 +88,7 @@ describe('Resources', () => {
       priority: 0
     };
     // Task 6 is below viewport and priority 0
-    let task_vpd_p1 = {
+    const task_vpd_p1 = {
       resource: {
         getLayoutBox() {
           return layoutRectLtwh(0, 600, 300, 50);
@@ -111,12 +111,12 @@ describe('Resources', () => {
 
   it('should calculate correct calcTaskTimeout', () => {
     // Task 1 is priority 0
-    let task_p0 = {
+    const task_p0 = {
       priority: 0,
       startTime: 0
     };
     // Task 2 is priority 1
-    let task_p1 = {
+    const task_p1 = {
       priority: 1,
       startTime: 0
     };
@@ -150,7 +150,7 @@ describe('Resources', () => {
 
   it('should not schedule non-prerenderable resource when' +
         ' document is hidden', () => {
-    let resource = {
+    const resource = {
       getState: () => {return ResourceState_.READY_FOR_LAYOUT;},
       isDisplayed: () => {return true;},
       isInViewport: () => {return true;},
@@ -164,7 +164,7 @@ describe('Resources', () => {
 
   it('should schedule prerenderable resource when' +
         ' document is hidden', () => {
-    let resource = {
+    const resource = {
       getState: () => {return ResourceState_.READY_FOR_LAYOUT;},
       isDisplayed: () => {return true;},
       isInViewport: () => {return true;},
@@ -181,7 +181,7 @@ describe('Resources', () => {
 
   it('should not schedule non-renderOutsideViewport resource when' +
         ' resource is not visible', () => {
-    let resource = {
+    const resource = {
       getState: () => {return ResourceState_.READY_FOR_LAYOUT;},
       isDisplayed: () => {return true;},
       isInViewport: () => {return false;},
@@ -195,7 +195,7 @@ describe('Resources', () => {
 
   it('should schedule renderOutsideViewport resource when' +
         ' resource is not visible', () => {
-    let resource = {
+    const resource = {
       getState: () => {return ResourceState_.READY_FOR_LAYOUT;},
       isDisplayed: () => {return true;},
       isInViewport: () => {return false;},
@@ -236,7 +236,7 @@ describe('Resources discoverWork', () => {
   }
 
   function createResource(id, rect) {
-    let resource = new Resource(id, createElement(rect), resources);
+    const resource = new Resource(id, createElement(rect), resources);
     resource.state_ = ResourceState_.READY_FOR_LAYOUT;
     resource.layoutBox_ = rect;
     return resource;
@@ -365,7 +365,7 @@ describe('Resources changeHeight', () => {
   }
 
   function createResource(id, rect) {
-    let resource = new Resource(id, createElement(rect), resources);
+    const resource = new Resource(id, createElement(rect), resources);
     resource.state_ = ResourceState_.READY_FOR_LAYOUT;
     resource.layoutBox_ = rect;
     resource.changeHeight = sinon.spy();
@@ -401,7 +401,7 @@ describe('Resources changeHeight', () => {
   });
 
   it('should schedule separate requests', () => {
-    let fallback2 = () => {};
+    const fallback2 = () => {};
     resources.scheduleChangeHeight_(resource1, 111, false, null);
     resources.scheduleChangeHeight_(resource2, 222, true, fallback2);
 
@@ -418,7 +418,7 @@ describe('Resources changeHeight', () => {
   });
 
   it('should only schedule latest request for the same resource', () => {
-    let fallback1 = () => {};
+    const fallback1 = () => {};
     resources.scheduleChangeHeight_(resource1, 111, true, fallback1);
     resources.scheduleChangeHeight_(resource1, 222, false, null);
 
@@ -462,7 +462,7 @@ describe('Resources changeHeight', () => {
     });
 
     it('should NOT change height and calls fallback', () => {
-      let fallback = sinon.spy();
+      const fallback = sinon.spy();
       resources.scheduleChangeHeight_(resource1, 111, false, fallback);
       resources.mutateWork_();
       expect(resources.changeHeightRequests_.length).to.equal(0);
@@ -581,7 +581,7 @@ describe('Resources.TaskQueue', () => {
     queue.enqueue({id: 'B', v: 2});
     queue.enqueue({id: 'C', v: 1});
 
-    let task = queue.peek(task => 10 - task.v);
+    const task = queue.peek(task => 10 - task.v);
     expect(task.id).to.equal('B');
   });
 });
@@ -903,7 +903,7 @@ describe('Resources.Resource', () => {
 
     resource.state_ = ResourceState_.READY_FOR_LAYOUT;
     resource.layoutBox_ = {left: 11, top: 12, width: 10, height: 10};
-    let promise = resource.startLayout(true);
+    const promise = resource.startLayout(true);
     expect(resource.layoutPromise_).to.not.equal(null);
     expect(resource.getState()).to.equal(ResourceState_.LAYOUT_SCHEDULED);
 
@@ -922,7 +922,7 @@ describe('Resources.Resource', () => {
 
     resource.state_ = ResourceState_.READY_FOR_LAYOUT;
     resource.layoutBox_ = {left: 11, top: 12, width: 10, height: 10};
-    let promise = resource.startLayout(true);
+    const promise = resource.startLayout(true);
     expect(resource.layoutPromise_).to.not.equal(null);
     expect(resource.getState()).to.equal(ResourceState_.LAYOUT_SCHEDULED);
 
diff --git a/test/functional/test-size-list.js b/test/functional/test-size-list.js
index c735cf13c911..569b479ff19c 100644
--- a/test/functional/test-size-list.js
+++ b/test/functional/test-size-list.js
@@ -102,7 +102,7 @@ describe('SizeList construct', () => {
 
 describe('SizeList select', () => {
   it('should select default last option', () => {
-    let sizeList = new SizeList([
+    const sizeList = new SizeList([
         {mediaQuery: 'media1', size: '444px'},
         {mediaQuery: 'media2', size: '333px'},
         {mediaQuery: 'media3', size: '222px'},
@@ -114,7 +114,7 @@ describe('SizeList select', () => {
   });
 
   it('should select a matching option', () => {
-    let sizeList = new SizeList([
+    const sizeList = new SizeList([
         {mediaQuery: 'media1', size: '444px'},
         {mediaQuery: 'media2', size: '333px'},
         {mediaQuery: 'media3', size: '222px'},
@@ -129,7 +129,7 @@ describe('SizeList select', () => {
   });
 
   it('should select first matching option', () => {
-    let sizeList = new SizeList([
+    const sizeList = new SizeList([
         {mediaQuery: 'media1', size: '444px'},
         {mediaQuery: 'media2', size: '333px'},
         {mediaQuery: 'media3', size: '222px'},
diff --git a/test/functional/test-srcset.js b/test/functional/test-srcset.js
index 2a73430f6fed..14e068b7f903 100644
--- a/test/functional/test-srcset.js
+++ b/test/functional/test-srcset.js
@@ -23,8 +23,8 @@ describe('Srcset parseSrcset', () => {
     var res = parseSrcset(s);
     expect(res.sources_.length).to.equal(expected.length);
     for (let i = 0; i < expected.length; i++) {
-      let r = res.sources_[i];
-      let e = expected[i];
+      const r = res.sources_[i];
+      const e = expected[i];
       expect(r.url).to.equal(e.url);
       expect(r.width).to.equal(e.width);
       expect(r.dpr).to.equal(e.dpr);
@@ -244,7 +244,7 @@ describe('Srcset construct', () => {
 
 describe('Srcset select', () => {
   it('select by width', () => {
-    let srcset = new Srcset([
+    const srcset = new Srcset([
         {url: 'image-1000', width: 1000},
         {url: 'image-500', width: 500},
         {url: 'image-250', width: 250},
@@ -283,7 +283,7 @@ describe('Srcset select', () => {
   });
 
   it('select by dpr', () => {
-    let srcset = new Srcset([
+    const srcset = new Srcset([
         {url: 'image-3x', dpr: 3},
         {url: 'image-2x', dpr: 2},
         {url: 'image', dpr: 1}
diff --git a/test/functional/test-style.js b/test/functional/test-style.js
index 44ffe08241cd..112958186dc1 100644
--- a/test/functional/test-style.js
+++ b/test/functional/test-style.js
@@ -30,7 +30,7 @@ describe('Style', () => {
   });
 
   it('toggle', () => {
-    let element = document.createElement('div');
+    const element = document.createElement('div');
     st.toggle(element);
     expect(element.style.display).to.equal('none');
     st.toggle(element);
@@ -42,19 +42,19 @@ describe('Style', () => {
   });
 
   it('setStyle', () => {
-    let element = document.createElement('div');
+    const element = document.createElement('div');
     st.setStyle(element, 'width', '1px');
     expect(element.style.width).to.equal('1px');
   });
 
   it('setStyle with vendor prefix', () => {
-    let element = { style: { WebkitTransitionDuration: '' } };
+    const element = { style: { WebkitTransitionDuration: '' } };
     st.setStyle(element, 'transitionDuration', '1s', undefined, true);
     expect(element.style.WebkitTransitionDuration).to.equal('1s');
   });
 
   it('setStyles', () => {
-    let element = document.createElement('div');
+    const element = document.createElement('div');
     st.setStyles(element, {
       width: st.px(101),
       height: st.px(102)
@@ -81,15 +81,15 @@ describe('Style', () => {
   });
 
   it('camelCaseToTitleCase', () => {
-    let str = 'theQuickBrownFox';
+    const str = 'theQuickBrownFox';
     expect(st.camelCaseToTitleCase(str)).to.equal('TheQuickBrownFox');
   });
 
   describe('getVendorJsPropertyName', () => {
 
     it('no prefix', () => {
-      let element = { style: { transitionDuration: '' } };
-      let prop = st
+      const element = { style: { transitionDuration: '' } };
+      const prop = st
           .getVendorJsPropertyName(element.style, 'transitionDuration', true);
       expect(prop).to.equal('transitionDuration');
     });
@@ -107,29 +107,29 @@ describe('Style', () => {
     });
 
     it('Webkit', () => {
-      let element = { style: { WebkitTransitionDuration: '' } };
-      let prop = st
+      const element = { style: { WebkitTransitionDuration: '' } };
+      const prop = st
           .getVendorJsPropertyName(element.style, 'transitionDuration', true);
       expect(prop).to.equal('WebkitTransitionDuration');
     });
 
     it('Moz', () => {
-      let element = { style: { MozTransitionDuration: '' } };
-      let prop = st
+      const element = { style: { MozTransitionDuration: '' } };
+      const prop = st
           .getVendorJsPropertyName(element.style, 'transitionDuration', true);
       expect(prop).to.equal('MozTransitionDuration');
     });
 
     it('ms', () => {
-      let element = { style: { msTransitionDuration: '' } };
-      let prop = st
+      const element = { style: { msTransitionDuration: '' } };
+      const prop = st
           .getVendorJsPropertyName(element.style, 'transitionDuration', true);
       expect(prop).to.equal('msTransitionDuration');
     });
 
     it('O opera', () => {
-      let element = { style: { OTransitionDuration: '' } };
-      let prop = st
+      const element = { style: { OTransitionDuration: '' } };
+      const prop = st
           .getVendorJsPropertyName(element.style, 'transitionDuration', true);
       expect(prop).to.equal('OTransitionDuration');
     });
diff --git a/test/functional/test-swipe.js b/test/functional/test-swipe.js
index 2233d0995957..1a941ea575a2 100644
--- a/test/functional/test-swipe.js
+++ b/test/functional/test-swipe.js
@@ -304,7 +304,7 @@ describe('Swipe', () => {
     swipeX.touchEnd_({});
 
     expect(vsyncTasks.length).to.equal(1);
-    let mutator = vsyncTasks[0];
+    const mutator = vsyncTasks[0];
     vsyncTasks = [];
     let cont = false;
 
diff --git a/test/functional/test-timer.js b/test/functional/test-timer.js
index e1f4950f26dd..525f12e68be8 100644
--- a/test/functional/test-timer.js
+++ b/test/functional/test-timer.js
@@ -41,7 +41,7 @@ describe('Timer', () => {
   });
 
   it('delay', () => {
-    let handler = () => {};
+    const handler = () => {};
     windowMock.expects('setTimeout').withExactArgs(handler, 111)
         .returns(1).once();
     windowMock.expects('clearTimeout').never();
diff --git a/test/functional/test-transition.js b/test/functional/test-transition.js
index 13260f9f1add..13656ba3c85c 100644
--- a/test/functional/test-transition.js
+++ b/test/functional/test-transition.js
@@ -30,9 +30,9 @@ describe('Transition', () => {
   });
 
   it('all', () => {
-    let func1 = sinon.spy();
-    let func2 = sinon.spy();
-    let all = tr.all([func1, func2]);
+    const func1 = sinon.spy();
+    const func2 = sinon.spy();
+    const all = tr.all([func1, func2]);
 
     expect(func1.callCount).to.equal(0);
     expect(func2.callCount).to.equal(0);
@@ -57,9 +57,9 @@ describe('Transition', () => {
   });
 
   it('withCurve', () => {
-    let func1 = (time, complete) => `${time * 2};${complete}`;
-    let curve = time => 0.2;
-    let curved = tr.withCurve(func1, curve);
+    const func1 = (time, complete) => `${time * 2};${complete}`;
+    const curve = time => 0.2;
+    const curved = tr.withCurve(func1, curve);
 
     expect(curved(0, false)).to.equal('0.4;false');
     expect(curved(0.5, false)).to.equal('0.4;false');
@@ -68,8 +68,8 @@ describe('Transition', () => {
   });
 
   it('setStyles', () => {
-    let element = document.createElement('div');
-    let func = tr.setStyles(element, {
+    const element = document.createElement('div');
+    const func = tr.setStyles(element, {
       width: tr.px(function(n) {return n * 100 + 1;}),
       height: tr.px(function(n) {return n * 100 + 2;}),
     });
@@ -126,7 +126,7 @@ describe('Transition', () => {
   });
 
   it('px', () => {
-    let func = tr.px(tr.numeric(0, 10));
+    const func = tr.px(tr.numeric(0, 10));
     expect(func(0)).to.equal('0px');
     expect(func(0.3)).to.equal('3px');
     expect(func(0.6)).to.equal('6px');
diff --git a/test/functional/test-uuid.js b/test/functional/test-uuid.js
index 3e2374ae6bce..64f0ba8ebd8c 100644
--- a/test/functional/test-uuid.js
+++ b/test/functional/test-uuid.js
@@ -19,7 +19,7 @@ import {randomUUID} from '../../src/uuid';
 
 describe('UUID', () => {
   it('should generate randomUUID with right pattern', () => {
-    let uuid = randomUUID();
+    const uuid = randomUUID();
     expect(uuid.indexOf('-', 0)).to.equal(8);
     expect(uuid.indexOf('-', 9)).to.equal(13);
     expect(uuid.indexOf('-', 14)).to.equal(18);
@@ -34,8 +34,8 @@ describe('UUID', () => {
     // 0         1         2         3
     // 012345678901234567890123456789012345
     // aaaaaaaa-aaaa-4aaa-baaa-aaaaaaaaaaaa
-    let v = randomUUID().substring(19, 20);
-    let n = parseInt(v, 16);
+    const v = randomUUID().substring(19, 20);
+    const n = parseInt(v, 16);
     expect(n & 0x8).to.not.equal(0, '|0x8');
     expect((n - 0x8) & ~0x3).to.equal(0, '&0x3');
   });
diff --git a/test/functional/test-viewer.js b/test/functional/test-viewer.js
index 964c45bfdf49..20e3a2acf800 100644
--- a/test/functional/test-viewer.js
+++ b/test/functional/test-viewer.js
@@ -56,7 +56,7 @@ describe('Viewer', () => {
         '&scrollTop=15';
     windowApi.location.hash = '#width=111&paddingTop=17&other=something';
     windowApi.document = {body: {style: {}}};
-    let viewer = new Viewer(windowApi);
+    const viewer = new Viewer(windowApi);
     expect(viewer.getViewportType()).to.equal('virtual');
     expect(viewer.getViewportWidth()).to.equal(111);
     expect(viewer.getViewportHeight()).to.equal(333);
@@ -77,7 +77,7 @@ describe('Viewer', () => {
 
   it('should configure visibilityState and prerender', () => {
     windowApi.location.hash = '#visibilityState=hidden&prerenderSize=3';
-    let viewer = new Viewer(windowApi);
+    const viewer = new Viewer(windowApi);
     expect(viewer.getVisibilityState()).to.equal('hidden');
     expect(viewer.isVisible()).to.equal(false);
     expect(viewer.getPrerenderSize()).to.equal(3);
@@ -86,11 +86,11 @@ describe('Viewer', () => {
   it('should configure correctly for iOS embedding', () => {
     windowApi.name = '__AMP__viewportType=natural';
     windowApi.parent = {};
-    let body = {style: {}};
-    let documentElement = {style: {}};
+    const body = {style: {}};
+    const documentElement = {style: {}};
     windowApi.document = {body: body, documentElement: documentElement};
     sandbox.mock(platform).expects('isIos').returns(true).once();
-    let viewer = new Viewer(windowApi);
+    const viewer = new Viewer(windowApi);
 
     expect(viewer.getViewportType()).to.equal('natural-ios-embed');
   });
@@ -98,8 +98,8 @@ describe('Viewer', () => {
   it('should NOT configure for iOS embedding if not embedded', () => {
     windowApi.name = '__AMP__viewportType=natural';
     windowApi.parent = windowApi;
-    let body = {style: {}};
-    let documentElement = {style: {}};
+    const body = {style: {}};
+    const documentElement = {style: {}};
     windowApi.document = {body: body, documentElement: documentElement};
     sandbox.mock(platform).expects('isIos').returns(true).once();
     expect(new Viewer(windowApi).getViewportType()).to.equal('natural');
@@ -144,7 +144,7 @@ describe('Viewer', () => {
 
   it('should post documentLoaded event', () => {
     viewer.postDocumentReady(11, 12);
-    let m = viewer.messageQueue_[0];
+    const m = viewer.messageQueue_[0];
     expect(m.eventType).to.equal('documentLoaded');
     expect(m.data.width).to.equal(11);
     expect(m.data.height).to.equal(12);
@@ -152,7 +152,7 @@ describe('Viewer', () => {
 
   it('should post documentResized event', () => {
     viewer.postDocumentResized(13, 14);
-    let m = viewer.messageQueue_[0];
+    const m = viewer.messageQueue_[0];
     expect(m.eventType).to.equal('documentResized');
     expect(m.data.width).to.equal(13);
     expect(m.data.height).to.equal(14);
@@ -171,7 +171,7 @@ describe('Viewer', () => {
     viewer.postDocumentResized(15, 16);
     expect(viewer.messageQueue_.length).to.equal(2);
     expect(viewer.messageQueue_[0].eventType).to.equal('documentLoaded');
-    let m = viewer.messageQueue_[1];
+    const m = viewer.messageQueue_[1];
     expect(m.eventType).to.equal('documentResized');
     expect(m.data.width).to.equal(15);
     expect(m.data.height).to.equal(16);
@@ -182,7 +182,7 @@ describe('Viewer', () => {
     viewer.postDocumentResized(13, 14);
     expect(viewer.messageQueue_.length).to.equal(2);
 
-    let delivered = [];
+    const delivered = [];
     viewer.setMessageDeliverer((eventType, data) => {
       delivered.push({eventType: eventType, data: data});
     });
diff --git a/test/functional/test-viewport.js b/test/functional/test-viewport.js
index fede11ed735a..2fed7491f758 100644
--- a/test/functional/test-viewport.js
+++ b/test/functional/test-viewport.js
@@ -120,7 +120,7 @@ describe('Viewport', () => {
   });
 
   it('should call binding.updateViewerViewport', () => {
-    let bindingMock = sandbox.mock(binding);
+    const bindingMock = sandbox.mock(binding);
     bindingMock.expects('updateViewerViewport').once();
     viewerViewportHandler();
     bindingMock.verify();
@@ -173,15 +173,15 @@ describe('Viewport', () => {
   });
 
   it('should update scroll pos and reset cache', () => {
-    let bindingMock = sandbox.mock(binding);
+    const bindingMock = sandbox.mock(binding);
     bindingMock.expects('setScrollTop').withArgs(117).once();
     viewport.setScrollTop(117);
     expect(viewport./*OK*/scrollTop_).to.be.null;
   });
 
   it('should change scrollTop for scrollIntoView and respect padding', () => {
-    let element = document.createElement('div');
-    let bindingMock = sandbox.mock(binding);
+    const element = document.createElement('div');
+    const bindingMock = sandbox.mock(binding);
     bindingMock.expects('getLayoutRect').withArgs(element)
         .returns({top: 111}).once();
     bindingMock.expects('setScrollTop').withArgs(111 - /* padding */ 19).once();
@@ -189,7 +189,7 @@ describe('Viewport', () => {
   });
 
   it('should deletegate scrollWidth', () => {
-    let bindingMock = sandbox.mock(binding);
+    const bindingMock = sandbox.mock(binding);
     bindingMock.expects('getScrollWidth').withArgs().returns(111).once();
     expect(viewport.getScrollWidth()).to.equal(111);
   });
@@ -247,7 +247,7 @@ describe('Viewport META', () => {
           .to.equal('width=device-width');
     });
     it('should stringify two key-values', () => {
-      let res = stringifyViewportMeta({
+      const res = stringifyViewportMeta({
         'width': 'device-width',
         'minimum-scale': '1'
       });
@@ -256,7 +256,7 @@ describe('Viewport META', () => {
           .to.be.true;
     });
     it('should stringify empty values', () => {
-      let res = stringifyViewportMeta({
+      const res = stringifyViewportMeta({
         'width': 'device-width',
         'minimal-ui': ''
       });
@@ -495,7 +495,7 @@ describe('ViewportBindingNatural', () => {
         clientHeight: 222
       }
     };
-    let size = binding.getSize();
+    const size = binding.getSize();
     expect(size.width).to.equal(111);
     expect(size.height).to.equal(222);
   });
@@ -541,12 +541,12 @@ describe('ViewportBindingNatural', () => {
     windowApi.pageXOffset = 100;
     windowApi.pageYOffset = 200;
     windowApi.document = {scrollingElement: {}};
-    let el = {
+    const el = {
       getBoundingClientRect: () => {
         return {left: 11.5, top: 12.5, width: 13.5, height: 14.5};
       }
     };
-    let rect = binding.getLayoutRect(el);
+    const rect = binding.getLayoutRect(el);
     expect(rect.left).to.equal(112);  // round(100 + 11.5)
     expect(rect.top).to.equal(213);  // round(200 + 12.5)
     expect(rect.width).to.equal(14);  // round(13.5)
@@ -633,8 +633,8 @@ describe('ViewportBindingNaturalIosEmbed', () => {
   });
 
   it('should setup document for embed scrolling', () => {
-    let documentElement = windowApi.document.documentElement;
-    let body = windowApi.document.body;
+    const documentElement = windowApi.document.documentElement;
+    const body = windowApi.document.body;
     expect(documentElement.style.overflow).to.equal('auto');
     expect(documentElement.style.webkitOverflowScrolling).to.equal('touch');
     expect(body.style.overflow).to.equal('auto');
@@ -676,7 +676,7 @@ describe('ViewportBindingNaturalIosEmbed', () => {
   it('should calculate size', () => {
     windowApi.innerWidth = 111;
     windowApi.innerHeight = 222;
-    let size = binding.getSize();
+    const size = binding.getSize();
     expect(size.width).to.equal(111);
     expect(size.height).to.equal(222);
   });
@@ -690,7 +690,7 @@ describe('ViewportBindingNaturalIosEmbed', () => {
   });
 
   it('should update scroll position via moving element', () => {
-    let moveEl = bodyChildren[1];
+    const moveEl = bodyChildren[1];
     binding.setScrollTop(17);
     expect(getStyle(moveEl, 'transform')).to.equal('translateY(17px)');
     expect(moveEl.scrollIntoView.callCount).to.equal(1);
@@ -702,12 +702,12 @@ describe('ViewportBindingNaturalIosEmbed', () => {
       return {top: -200, left: -100};
     };
     binding.onScrolled_();
-    let el = {
+    const el = {
       getBoundingClientRect: () => {
         return {left: 11.5, top: 12.5, width: 13.5, height: 14.5};
       }
     };
-    let rect = binding.getLayoutRect(el);
+    const rect = binding.getLayoutRect(el);
     expect(rect.left).to.equal(112);  // round(100 + 11.5)
     expect(rect.top).to.equal(213);  // round(200 + 12.5)
     expect(rect.width).to.equal(14);  // round(13.5)
@@ -715,7 +715,7 @@ describe('ViewportBindingNaturalIosEmbed', () => {
   });
 
   it('should set scroll position via moving element', () => {
-    let moveEl = bodyChildren[1];
+    const moveEl = bodyChildren[1];
     binding.setScrollPos_(10);
     expect(getStyle(moveEl, 'transform')).to.equal('translateY(10px)');
     expect(moveEl.scrollIntoView.callCount).to.equal(1);
@@ -723,10 +723,10 @@ describe('ViewportBindingNaturalIosEmbed', () => {
   });
 
   it('should adjust scroll position when scrolled to 0', () => {
-    let posEl = bodyChildren[0];
+    const posEl = bodyChildren[0];
     posEl.getBoundingClientRect = () => {return {top: 0, left: 0};};
-    let moveEl = bodyChildren[1];
-    let event = {preventDefault: sinon.spy()};
+    const moveEl = bodyChildren[1];
+    const event = {preventDefault: sinon.spy()};
     binding.adjustScrollPos_(event);
     expect(getStyle(moveEl, 'transform')).to.equal('translateY(1px)');
     expect(moveEl.scrollIntoView.callCount).to.equal(1);
@@ -735,28 +735,28 @@ describe('ViewportBindingNaturalIosEmbed', () => {
   });
 
   it('should adjust scroll position when scrolled to 0; w/o event', () => {
-    let posEl = bodyChildren[0];
+    const posEl = bodyChildren[0];
     posEl.getBoundingClientRect = () => {return {top: 0, left: 0};};
-    let moveEl = bodyChildren[1];
+    const moveEl = bodyChildren[1];
     binding.adjustScrollPos_();
     expect(moveEl.scrollIntoView.callCount).to.equal(1);
   });
 
   it('should NOT adjust scroll position when scrolled away from 0', () => {
-    let posEl = bodyChildren[0];
+    const posEl = bodyChildren[0];
     posEl.getBoundingClientRect = () => {return {top: -10, left: 0};};
-    let moveEl = bodyChildren[1];
-    let event = {preventDefault: sinon.spy()};
+    const moveEl = bodyChildren[1];
+    const event = {preventDefault: sinon.spy()};
     binding.adjustScrollPos_(event);
     expect(moveEl.scrollIntoView.callCount).to.equal(0);
     expect(event.preventDefault.callCount).to.equal(0);
   });
 
   it('should NOT adjust scroll position when overscrolled', () => {
-    let posEl = bodyChildren[0];
+    const posEl = bodyChildren[0];
     posEl.getBoundingClientRect = () => {return {top: 10, left: 0};};
-    let moveEl = bodyChildren[1];
-    let event = {preventDefault: sinon.spy()};
+    const moveEl = bodyChildren[1];
+    const event = {preventDefault: sinon.spy()};
     binding.adjustScrollPos_(event);
     expect(moveEl.scrollIntoView.callCount).to.equal(0);
     expect(event.preventDefault.callCount).to.equal(0);
@@ -811,7 +811,7 @@ describe('ViewportBindingVirtual', () => {
   });
 
   it('should send event on scroll changed', () => {
-    let scrollHandler = sinon.spy();
+    const scrollHandler = sinon.spy();
     binding.onScroll(scrollHandler);
     expect(scrollHandler.callCount).to.equal(0);
 
@@ -834,7 +834,7 @@ describe('ViewportBindingVirtual', () => {
   });
 
   it('should send event on size changed', () => {
-    let resizeHandler = sinon.spy();
+    const resizeHandler = sinon.spy();
     binding.onResize(resizeHandler);
     expect(resizeHandler.callCount).to.equal(0);
 
@@ -867,12 +867,12 @@ describe('ViewportBindingVirtual', () => {
   });
 
   it('should NOT offset client rect for layout', () => {
-    let el = {
+    const el = {
       getBoundingClientRect: () => {
         return {left: 11.5, top: 12.5, width: 13.5, height: 14.5};
       }
     };
-    let rect = binding.getLayoutRect(el);
+    const rect = binding.getLayoutRect(el);
     expect(rect.left).to.equal(12);  // round(11.5)
     expect(rect.top).to.equal(13);  // round(12.5)
     expect(rect.width).to.equal(14);  // round(13.5)
diff --git a/test/integration/test-amp-carousel.js b/test/integration/test-amp-carousel.js
index 93ae73c1987c..049e58020f8a 100644
--- a/test/integration/test-amp-carousel.js
+++ b/test/integration/test-amp-carousel.js
@@ -49,11 +49,11 @@ describe('integration amp-carousel', () => {
     it.skip('should only have the next button visible' +
        'when on first item', () => {
       return fixture.awaitEvent('amp:load:start', 1).then(() => {
-        let amp = fixture.doc.querySelector('#carousel-1');
+        const amp = fixture.doc.querySelector('#carousel-1');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.visible;
       });
@@ -61,11 +61,11 @@ describe('integration amp-carousel', () => {
 
     it.skip('should not be able to go past the first or last item', () => {
       return fixture.awaitEvent('amp:load:start', 1).then(() => {
-        let amp = fixture.doc.querySelector('#carousel-1');
+        const amp = fixture.doc.querySelector('#carousel-1');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         prevBtn.style.visibility = 'visible';
         nextBtn.style.visibility = 'visible';
         expect(prevBtn).to.be.visible;
@@ -88,11 +88,11 @@ describe('integration amp-carousel', () => {
     it.skip('(type=slide) should only have the next button visible when on ' +
        'first item', () => {
       return fixture.awaitEvent('amp:load:start', 4).then(() => {
-        let amp = fixture.doc.querySelector('#carousel-4');
+        const amp = fixture.doc.querySelector('#carousel-4');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.visible;
       });
@@ -101,10 +101,10 @@ describe('integration amp-carousel', () => {
     it.skip('should only have the prev button visible' +
        'when on last item', () => {
       return fixture.awaitEvent('amp:load:start', 1).then(() => {
-        let amp = fixture.doc.querySelector('#carousel-1');
+        const amp = fixture.doc.querySelector('#carousel-1');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.visible;
         nextBtn.click();
@@ -118,10 +118,10 @@ describe('integration amp-carousel', () => {
     it.skip('(type=slides) should only have the prev button visible when ' +
        'on last item', () => {
       return fixture.awaitEvent('amp:load:start', 4).then(() => {
-        let amp = fixture.doc.querySelector('#carousel-4');
+        const amp = fixture.doc.querySelector('#carousel-4');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.visible;
         nextBtn.click();
@@ -138,10 +138,10 @@ describe('integration amp-carousel', () => {
     it.skip('(type=slides loop) should always have a prev and next button be ' +
        'able to get past the first and last item', () => {
       return fixture.awaitEvent('amp:load:start', 7).then(() => {
-        let amp = fixture.doc.querySelector('#carousel-7');
+        const amp = fixture.doc.querySelector('#carousel-7');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.not.have.class('-amp-disabled');
         expect(nextBtn).to.not.have.class('-amp-disabled');
         nextBtn.click();
@@ -167,11 +167,11 @@ describe('integration amp-carousel', () => {
        'item', () => {
       return fixture.awaitEvent('amp:load:start', 2).then(() => {
         fixture.doc.body.classList.add('amp-mode-mouse');
-        let amp = fixture.doc.querySelector('#carousel-2');
+        const amp = fixture.doc.querySelector('#carousel-2');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.hidden;
         expect(prevBtn).to.have.class('amp-disabled');
@@ -183,11 +183,11 @@ describe('integration amp-carousel', () => {
        'only a single item', () => {
       return fixture.awaitEvent('amp:load:start', 5).then(() => {
         fixture.doc.body.classList.add('amp-mode-mouse');
-        let amp = fixture.doc.querySelector('#carousel-5');
+        const amp = fixture.doc.querySelector('#carousel-5');
         expect(fixture.doc.body).to.have.class('amp-mode-mouse');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.hidden;
         expect(prevBtn).to.have.class('amp-disabled');
@@ -201,11 +201,11 @@ describe('integration amp-carousel', () => {
     it.skip('should not have the buttons visible', () => {
       return fixture.awaitEvent('amp:load:start', 1).then(() => {
         fixture.doc.body.classList.remove('amp-mode-mouse');
-        let amp = fixture.doc.querySelector('#carousel-1');
+        const amp = fixture.doc.querySelector('#carousel-1');
         expect(fixture.doc.body).to.not.have.class('amp-mode-mouse');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.hidden;
       });
@@ -214,11 +214,11 @@ describe('integration amp-carousel', () => {
     it.skip('(type=slides) should not have the buttons visible', () => {
       return fixture.awaitEvent('amp:load:start', 4).then(() => {
         fixture.doc.body.classList.remove('amp-mode-mouse');
-        let amp = fixture.doc.querySelector('#carousel-4');
+        const amp = fixture.doc.querySelector('#carousel-4');
         expect(fixture.doc.body).to.not.have.class('amp-mode-mouse');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.hidden;
       });
@@ -231,14 +231,14 @@ describe('integration amp-carousel', () => {
        'is not on body', () => {
       return fixture.awaitEvent('amp:load:start', 3).then(() => {
         fixture.doc.body.classList.remove('amp-mode-mouse');
-        let amp = fixture.doc.querySelector('#carousel-3');
+        const amp = fixture.doc.querySelector('#carousel-3');
         expect(fixture.doc.body).to.not.have.class('amp-mode-mouse');
 
         expect(amp).to.have.attribute('controls');
         expect(amp).to.have.class('-amp-carousel-has-controls');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.visible;
       });
@@ -248,14 +248,14 @@ describe('integration amp-carousel', () => {
        'even when `amp-mode-mouse` is not on body', () => {
       return fixture.awaitEvent('amp:load:start', 6).then(() => {
         fixture.doc.body.classList.remove('amp-mode-mouse');
-        let amp = fixture.doc.querySelector('#carousel-6');
+        const amp = fixture.doc.querySelector('#carousel-6');
         expect(fixture.doc.body).to.not.have.class('amp-mode-mouse');
 
         expect(amp).to.have.attribute('controls');
         expect(amp).to.have.class('-amp-carousel-has-controls');
 
-        let prevBtn = amp.querySelector('.amp-carousel-button-prev');
-        let nextBtn = amp.querySelector('.amp-carousel-button-next');
+        const prevBtn = amp.querySelector('.amp-carousel-button-prev');
+        const nextBtn = amp.querySelector('.amp-carousel-button-next');
         expect(prevBtn).to.be.hidden;
         expect(nextBtn).to.be.visible;
       });
diff --git a/testing/iframe.js b/testing/iframe.js
index 668a31a80692..6389cb6490c0 100644
--- a/testing/iframe.js
+++ b/testing/iframe.js
@@ -86,7 +86,7 @@ export function createFixtureIframe(fixture, initialIframeHeight, done) {
         });
       };
       // Record firing of custom events.
-      for (let name in events) {
+      for (const name in events) {
         win.addEventListener(name, () => {
           events[name]++;
         });