Give ElementInternals element-reflecting attributes more power

In particular allow them to point to any known element, as long as that has the same shadow-including root as the ElementInternals's element. This is not considered to break encapsulation as ElementInternals itself is encapsulated.

Tests: anyone want to volunteer?

Fixes #8544.

source

@@ -8147,10 +8147,22 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
       null:</p>
 
       <ul>
-       <li><p>If <var>reflectedTarget</var>'s <span>explicitly set <var>attr</var>-element</span> is
-       a <span>descendant</span> of any of <var>element</var>'s <span
-       data-x="concept-shadow-including-ancestor">shadow-including ancestors</span>, then return
-       <var>reflectedTarget</var>'s <span>explicitly set <var>attr</var>-element</span>.</p></li>
+       <li>
+        <p>If one of the following is true:
+
+        <ul class="brief">
+         <li><var>reflectedTarget</var> is an <code>ElementInternals</code> object and
+         <var>reflectedTarget</var>'s <span>explicitly set <var>attr</var>-element</span>'s
+         <span>shadow-including root</span> is <var>element</var>'s <span>shadow-including
+         root</span></li>
+         <li><var>reflectedTarget</var>'s <span>explicitly set <var>attr</var>-element</span> is a
+         <span>descendant</span> of any of <var>element</var>'s <span
+         data-x="concept-shadow-including-ancestor">shadow-including ancestors</span></li>
+        </ul>
+
+        <p>then return <var>reflectedTarget</var>'s <span>explicitly set
+        <var>attr</var>-element</span>.</p>
+       </li>
 
        <li><p>Return null.</p></li>
       </ul>
@@ -8257,16 +8269,20 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
       <ol>
        <li>
         <p><span data-x="list iterate">For each</span> <var>attrElement</var> in
-        <var>reflectedTarget</var>'s <span>explicitly set <var>attr</var>-elements</span>:</p>
+        <var>reflectedTarget</var>'s <span>explicitly set <var>attr</var>-elements</span>: if one of
+        the following is true:
 
-        <ol>
-         <li><p>If <var>attrElement</var> is not a <span>descendant</span> of any of
-         <var>element</var>'s <span data-x="concept-shadow-including-ancestor">shadow-including
-         ancestors</span>, then <span>continue</span>.</p></li>
+        <ul class="brief">
+         <li><var>reflectedTarget</var> is an <code>ElementInternals</code> object and
+         <var>attrElement</var>'s <span>shadow-including root</span> is <var>element</var>'s
+         <span>shadow-including root</span></li>
 
-         <li><p><span data-x="list append">Append</span> <var>attrElement</var> to
-         <var>elements</var>.</p></li>
-        </ol>
+         <li><var>attrElement</var> is a <span>descendant</span> of any of <var>element</var>'s
+         <span data-x="concept-shadow-including-ancestor">shadow-including ancestors</span></li>
+        </ul>
+
+        <p>then <span data-x="list append">append</span> <var>attrElement</var> to
+        <var>elements</var>.</p>
        </li>
       </ol>
      </li>