Add activation behavior for <summary>

Fixes #2246. This also fixes the value that the open attribute is set to
to be the empty string, instead of "open", matching existing
implementations.

Tests at web-platform-tests/wpt#4539 show that
both implementers of <details> follow these same semantics, despite
there having been no spec previously. The only exception is that Blink
included a being-rendered check, but that was removed; see
https://bugs.chromium.org/p/chromium/issues/detail?id=681711.

There's still some discussion ongoing as to whether we should change the
content models to disallow interactive descendants of <summary>
elements, happening in #2272.

source

@@ -55325,10 +55325,17 @@ interface <dfn>HTMLDetailsElement</dfn> : <span>HTMLElement</span> {
   information should be shown.</p>
 
   <p>The user agent should allow the user to request that the additional information be shown or
-  hidden. To honor a request for the details to be shown, the user agent must set the <code
-  data-x="attr-details-open">open</code> attribute on the element to the value <code
-  data-x="">open</code>. To honor a request for the information to be hidden, the user agent must
-  remove the <code data-x="attr-details-open">open</code> attribute from the element.</p>
+  hidden. To honor a request for the details to be shown, the user agent must <span
+  data-x="concept-element-attributes-set-value">set</span> the <code
+  data-x="attr-details-open">open</code> attribute on the element to the empty string. To honor a
+  request for the information to be hidden, the user agent must <span
+  data-x="concept-element-attributes-remove">remove</span> the <code
+  data-x="attr-details-open">open</code> attribute from the element.</p>
+
+  <p class="note">This ability to request that additional information be shown or hidden may simply
+  be the <span>activation behavior</span> of the appropriate <code>summary</code> element, in the
+  case such an element exists. However, if no such element exists, user agents should still provide
+  this ability through some other user interface affordance.</p>
 
   <p>Whenever the <code data-x="attr-details-open">open</code> attribute is added to or removed from
   a <code>details</code> element, the user agent must <span>queue a task</span> that runs the
@@ -55446,6 +55453,33 @@ interface <dfn>HTMLDetailsElement</dfn> : <span>HTMLElement</span> {
   rest of the contents of the <code>summary</code> element's parent <code>details</code>
   element<span w-nodev>, if any</span>.</p>
 
+  <div w-nodev>
+
+  <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 has no parent node, then abort these steps.</p></li>
+
+   <li><p>Let <var>parent</var> be this <code>summary</code> element's parent node.</p>
+
+   <li><p>If <var>parent</var> is not a <code>details</code> element, then abort these steps.</p>
+
+   <li><p>If <var>parent</var>'s first child <code>summary</code> element is not this
+   <code>summary</code> element, then abort these steps.</p></li>
+
+   <li>
+    <p>If the <code data-x="attr-details-open">open</code> attribute is present on
+    <var>parent</var>, then <span data-x="concept-element-attributes-remove">remove</span> it.
+    Otherwise, <span data-x="concept-element-attributes-set-value">set</span> <var>parent</var>'s
+    <code data-x="attr-details-open">open</code> attribute to the empty string.</p>
+
+    <p class="note">This will then run the <span>details notification task steps</span>.</p>
+   </li>
+  </ol>
+
+  </div>
+
 
 
   <h4 id="the-menu-element">The <dfn id="menus"><code>menu</code></dfn> element</h4>
@@ -72262,6 +72296,9 @@ END:VCARD</pre>
 
      <li><code>menuitem</code> elements</li>
 
+     <li><code>summary</code> elements that are the first child <code>summary</code> element of a
+     <code>details</code> element</li>
+
      <li>Elements with a <code data-x="attr-draggable">draggable</code> attribute set, if that would
      enable the user agent to allow the user to begin a drag operations for those elements without
      the use of a pointing device</li>