Add shadow host considerations for the :focus selector

Part of #2013. Note that the delegates focus flag is not consulted.

Tests: web-platform-tests/wpt#17493

source

@@ -3175,7 +3175,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#concept-node-document">node document</dfn> concept</li>
      <li><dfn data-x="concept-document-type" data-x-href="https://dom.spec.whatwg.org/#concept-document-type">document type</dfn> concept</li>
      <li><dfn data-x="concept-DocumentFragment-host" data-x-href="https://dom.spec.whatwg.org/#concept-documentfragment-host">host</dfn> concept</li>
-     <li><dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root</dfn> concept</li>
+     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root</dfn> concept</li>
 
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#interface-htmlcollection"><code>HTMLCollection</code></dfn> interface</li>
      <li><dfn data-x="dom-HTMLCollection-length" data-x-href="https://dom.spec.whatwg.org/#dom-htmlcollection-length"><code>HTMLCollection.length</code></dfn> attribute</li>
@@ -69014,11 +69014,23 @@ Demos:
    <dt><dfn data-noexport=""><code data-x="selector-focus">:focus</code></dfn></dt>
    <dd>
     <p>For the purposes of the CSS <code data-x="selector-focus">:focus</code>
-    <span>pseudo-class</span>, an <dfn>element has the focus</dfn> when its <span>top-level browsing
-    context</span> has the system focus, it is not itself a <span>browsing context container</span>,
-    and it is one of the elements listed in the <span>focus chain</span> of the <span
-    data-x="currently focused area of a top-level browsing context">currently focused area of the
-    top-level browsing context</span>.</p>
+    <span>pseudo-class</span>, an <dfn>element has the focus</dfn> when:</p>
+
+    <ul>
+     <li><p>its <span>top-level browsing context</span> has the system focus;</p></li>
+     <li><p>it is not itself a <span>browsing context container</span>; and</p></li>
+     <li>
+      <p>at least one of the following is true:</p>
+      <ul>
+       <li><p>it is one of the elements listed in the <span>focus chain</span> of the <span
+       data-x="currently focused area of a top-level browsing context">currently focused area of
+       the top-level browsing context</span>, or</p></li>
+       <li><p>its <span data-x="concept-element-shadow-root">shadow root</span>
+       <var>shadowRoot</var> is not null and <var>shadowRoot</var> is the <span>root</span> of at
+       least one element that <span data-x="element has the focus">has the focus</span>.</p></li>
+      </ul>
+     </li>
+    </ul>
    </dd>
 
    <dt><dfn data-noexport=""><code data-x="selector-target">:target</code></dfn></dt>
@@ -124681,6 +124693,7 @@ INSERT INTERFACES HERE
   Rahul Purohit,
   Raj Doshi,
   Rajas Moonka,
+  Rakina Zata Amni,
   Ralf Stoltze,
   Ralph Giles,
   Raphael Champeimont,