Define what tabIndex returns when the tabindex attribute is not set

This updates the tabIndex getter's default value to be 0 for a specific
list of element types, instead of the spec's previous "elements that are
focusable". This matches reality better; apart from minor edge cases
all three browser engines match this spec in observable behavior.

Closes whatwg#1786. Closes whatwg#4464. See the focus meta-bug in whatwg#4607 for related
discussions, including on adding an API that actually reflects whether
an element is focusable (like the tabIndex getter was kind-of supposed
to do).

Tests: web-platform-tests/wpt#17657

source

@@ -57394,18 +57394,30 @@ interface <dfn>HTMLDetailsElement</dfn> : <span>HTMLElement</span> {
 
   <div w-nodev>
 
-  <p>The <span>activation behavior</span> of <code>summary</code> elements is to run the following
-  steps:</p>
+  <p>A <code>summary</code> element is a <dfn>summary for its parent details</dfn> if the following
+  algorithm returns true:</p>
 
   <ol>
-   <li><p>If this <code>summary</code> element has no parent node, then return.</p></li>
+   <li><p>If this <code>summary</code> element has no parent, then return false.</p></li>
 
-   <li><p>Let <var>parent</var> be this <code>summary</code> element's parent node.</p>
+   <li><p>Let <var>parent</var> be this <code>summary</code> element's parent.</p></li>
 
-   <li><p>If <var>parent</var> is not a <code>details</code> element, then return.</p>
+   <li><p>If <var>parent</var> is not a <code>details</code> element, then return false.</p></li>
 
    <li><p>If <var>parent</var>'s first <code>summary</code> element child is not this
-   <code>summary</code> element, then return.</p></li>
+   <code>summary</code> element, then return false.</p></li>
+
+   <li><p>Return true.</p></li>
+  </ol>
+
+  <p>The <span>activation behavior</span> of <code>summary</code> elements is to run the following
+  steps:</p>
+
+  <ol>
+   <li><p>If this <code>summary</code> element is not the <span>summary for its parent
+   details</span>, then return.</p></li>
+
+   <li><p>Let <var>parent</var> be this <code>summary</code> element's parent.</p></li>
 
    <li>
     <p>If the <code data-x="attr-details-open">open</code> attribute is present on
@@ -73819,8 +73831,12 @@ END:VCARD</pre>
 
   <p>The <dfn><code data-x="dom-tabIndex">tabIndex</code></dfn> IDL attribute must
   <span>reflect</span> the value of the <code data-x="attr-tabindex">tabindex</code> content
-  attribute. Its default value is 0 for elements that are focusable and &#x2212;1 for elements that
-  are not focusable.</p>
+  attribute. The default value is 0 if the element is an <code>a</code>, <code>area</code>,
+  <code>button</code>, <code>iframe</code>, <code>input</code>, <code>select</code>, or
+  <code>textarea</code> element, or is a <code>summary</code> element that is a <span>summary for
+  its parent details</span>. The default value is &#x2212;1 otherwise.</p>
+
+  <p class="note">The varying default value based on element type is a historical artifact.</p>
 
   </div>