Fix the set an attribute algorithm

The concept to replace an attribute is introduced, avoiding two mutation records
to be queued in case an attribute already existed in the element when setting
an attribute in it.

dom.bs

@@ -5151,6 +5151,26 @@ run these steps:
  <li>An <a>attribute is removed</a>.
 </ol>
 
+To <dfn export id=concept-element-attributes-replace lt="replace an attribute">replace</dfn> an
+<a>attribute</a> <var>oldAttr</var> by an <a>attribute</a> <var>newAttr</var>
+in an <a for="/">element</a> <var>element</var>, run these steps:
+
+<ol>
+ <li><p><a>Queue a mutation record</a> of "<code>attributes</code>" for <var>element</var>
+ with name <var>oldAttr</var>'s <a for=Attr>local name</a>,
+ namespace <var>oldAttr</var>'s <a for=Attr>namespace</a>,
+ and oldValue <var>oldAttr</var>'s <a for=Attr>value</a>.
+
+ <li><p>Replace <var>oldAttr</var> by <var>newAttr</var> in the <var>element</var>'s
+ <a for=Element>attribute list</a>.
+
+ <li><p>Set <var>oldAttr</var>'s <a for=Attr>element</a> to null.
+
+ <li><p>Set <var>newAttr</var>'s <a for=Attr>element</a> to <var>element</var>.
+
+ <li><p>An <a>attribute is set</a> and an <a>attribute is changed</a>.
+</ol>
+
 <hr>
 
 To <dfn export id=concept-element-attributes-get-by-name>get an attribute by name</dfn> given a
@@ -5194,12 +5214,12 @@ To <dfn export id=concept-element-attributes-set>set an attribute</dfn> given an
 
  <li><p>If <var>oldAttr</var> is <var>attr</var>, return <var>attr</var>.
 
- <li><p>If <var>oldAttr</var> is non-null, <a lt="remove an attribute">remove</a> it from
- <var>element</var>.
+ <li><p>If <var>oldAttr</var> is non-null, <a lt="replace an attribute">replace</a> it
+  by <var>attr</var> in <var>element</var>.
 
- <li><p><a lt="append an attribute">Append</a> <var>attr</var> to <var>element</var>.
+ <li><p>Otherwise, <a lt="append an attribute">append</a> <var>attr</var> to <var>element</var>.
 
- <li>Return <var>oldAttr</var>.
+ <li><p>Return <var>oldAttr</var>.
 </ol>
 
 To <dfn export id=concept-element-attributes-set-value>set an attribute value</dfn> for

dom.html

@@ -2750,6 +2750,21 @@ <h3 class="heading settled" data-level="4.8" id="interface-element"><span class=
     <li>Set <var>attribute</var>’s <a data-link-type="dfn" href="#concept-attribute-element">element</a> to null. 
     <li>An <a data-link-type="dfn" href="#attribute-is-removed">attribute is removed</a>. 
    </ol>
+   <p>To <dfn data-dfn-type="dfn" data-export="" data-lt="replace an attribute" id="concept-element-attributes-replace">replace<a class="self-link" href="#concept-element-attributes-replace"></a></dfn> an <a data-link-type="dfn" href="#concept-attribute">attribute</a> <var>oldAttr</var> by an <a data-link-type="dfn" href="#concept-attribute">attribute</a> <var>newAttr</var> in an <a data-link-type="dfn" href="#concept-element">element</a> <var>element</var>, run these steps:</p>
+   <ol>
+    <li>
+     <p><a data-link-type="dfn" href="#queue-a-mutation-record">Queue a mutation record</a> of "<code>attributes</code>" for <var>element</var> with name <var>oldAttr</var>’s <a data-link-type="dfn" href="#concept-attribute-local-name">local name</a>,
+ namespace <var>oldAttr</var>’s <a data-link-type="dfn" href="#concept-attribute-namespace">namespace</a>,
+ and oldValue <var>oldAttr</var>’s <a data-link-type="dfn" href="#concept-attribute-value">value</a>. </p>
+    <li>
+     <p>Replace <var>oldAttr</var> by <var>newAttr</var> in the <var>element</var>’s <a data-link-type="dfn" href="#concept-element-attribute">attribute list</a>. </p>
+    <li>
+     <p>Set <var>oldAttr</var>’s <a data-link-type="dfn" href="#concept-attribute-element">element</a> to null. </p>
+    <li>
+     <p>Set <var>newAttr</var>’s <a data-link-type="dfn" href="#concept-attribute-element">element</a> to <var>element</var>. </p>
+    <li>
+     <p>An <a data-link-type="dfn" href="#attribute-is-set">attribute is set</a> and an <a data-link-type="dfn" href="#attribute-is-changed">attribute is changed</a>. </p>
+   </ol>
    <hr>
    <p>To <dfn data-dfn-type="dfn" data-export="" id="concept-element-attributes-get-by-name">get an attribute by name<a class="self-link" href="#concept-element-attributes-get-by-name"></a></dfn> given a <var>qualifiedName</var> and <a data-link-type="dfn" href="#concept-element">element</a> <var>element</var>, run these steps:</p>
    <ol>
@@ -2773,10 +2788,12 @@ <h3 class="heading settled" data-level="4.8" id="interface-element"><span class=
     <li>
      <p>If <var>oldAttr</var> is <var>attr</var>, return <var>attr</var>. </p>
     <li>
-     <p>If <var>oldAttr</var> is non-null, <a data-link-type="dfn" href="#concept-element-attributes-remove">remove</a> it from <var>element</var>. </p>
+     <p>If <var>oldAttr</var> is non-null, <a data-link-type="dfn" href="#concept-element-attributes-replace">replace</a> it
+  by <var>attr</var> in <var>element</var>. </p>
+    <li>
+     <p>Otherwise, <a data-link-type="dfn" href="#concept-element-attributes-append">append</a> <var>attr</var> to <var>element</var>. </p>
     <li>
-     <p><a data-link-type="dfn" href="#concept-element-attributes-append">Append</a> <var>attr</var> to <var>element</var>. </p>
-    <li>Return <var>oldAttr</var>. 
+     <p>Return <var>oldAttr</var>. </p>
    </ol>
    <p>To <dfn data-dfn-type="dfn" data-export="" id="concept-element-attributes-set-value">set an attribute value<a class="self-link" href="#concept-element-attributes-set-value"></a></dfn> for
 an <a data-link-type="dfn" href="#concept-element">element</a> <var>element</var> using a <var>localName</var> and <var>value</var>, and an optional <var>prefix</var>, and <var>namespace</var>, run these steps:</p>
@@ -5113,6 +5130,7 @@ <h3 class="no-num heading settled" id="index-defined-here"><span class="content"
    <li><a href="#dom-document-renamenode">renameNode()</a><span>, in §8.2</span>
    <li><a href="#concept-node-replace">replace</a><span>, in §4.2.1</span>
    <li><a href="#concept-node-replace-all">replace all</a><span>, in §4.2.1</span>
+   <li><a href="#concept-element-attributes-replace">replace an attribute</a><span>, in §4.8</span>
    <li><a href="#dom-node-replacechild">replaceChild(node, child)</a><span>, in §4.4</span>
    <li><a href="#concept-cd-replace">replace data</a><span>, in §4.9</span>
    <li><a href="#dom-characterdata-replacedata">replaceData(offset, count, data)</a><span>, in §4.9</span>