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. It fixes a piece of #1837.

source

@@ -78799,8 +78799,8 @@ dictionary <dfn dictionary>ToggleEventInit</dfn> : <span>EventInit</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>delegating its rendering to 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.
@@ -130792,6 +130792,11 @@ console.assert(container.firstChild instanceof SuperP);
   <p class="note">This specification does not define the precise timing for when the intersection is
   tested, but it is suggested that the timing match that of the Intersection Observer API. <ref>INTERSECTIONOBSERVER</ref></p>
 
+  <p>An element is <dfn>delegating its rendering to 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', or some equivalent in other styling languages.
+  <ref>CSSDISPLAY</ref></p>
+
   <hr>
 
   <p>User agents that do not honor author-level CSS style sheets are nonetheless expected to act as