Allow elements with display:contents to be focusable.

This aligns HTML with the conclusion in
w3c/csswg-drafts#2632 that elements with CSS
`display: contents` should be focusable just like normally-rendered
elements.

source

@@ -77538,8 +77538,8 @@ partial interface <span id="NavigatorUserActivation-partial">Navigator</span> {
 
       <li>the element is not <span>inert</span>;</li>
 
-      <li>the element is either <span>being rendered</span> or <span>being used as relevant canvas
-      fallback content</span>.</li>
+      <li>the element is either <span>being rendered</span>, <span>being rendered by its
+      children</span>, or <span>being used as relevant canvas fallback content</span>.</li>
      </ul>
      <td headers="td-fa-1 th-fa-dom-rep">
      The element itself.
@@ -123902,6 +123902,11 @@ console.assert(container.firstChild instanceof SuperP);
   tested, but it is suggested that the timing match that of the Intersection Observer API. <ref
   spec=INTERSECTIONOBSERVER></p>
 
+  <p>An element is <dfn>being rendered by its children</dfn> if it is not <span>being
+  rendered</span> but its children (if any) could <span data-x="being rendered">be rendered</span>,
+  as a result of CSS 'display: contents' <ref spec=CSSDISPLAY>, or some equivalent in other styling
+  languages.</p>
+
   <hr>
 
   <p>User agents that do not honor author-level CSS style sheets are nonetheless expected to act as