Adjust shadowrootcustomelementregistry attribute serialization

whatwg/dom#1409 changed where attachShadow() obtains its registry so the serialization should account for that as well.

Tests: web-platform-tests/wpt#56134.

Fixes #11892.

Author: annevk

source

@@ -3412,6 +3412,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-element-attributes-get-by-name">get an attribute by name</dfn> algorithm</li>
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#valid-attribute-local-name">valid attribute local name</dfn></li>
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#valid-element-local-name">valid element local name</dfn></li>
+     <li><dfn data-x-href="https://dom.spec.whatwg.org/#is-a-global-custom-element-registry">is a global custom element registry</dfn></li>
     </ul>
 
     <p>The following features are defined in <cite>UI Events</cite>: <ref>UIEVENTS</ref></p>
@@ -144140,10 +144141,29 @@ document.body.appendChild(text);
        <li><p>If <var>shadow</var>'s <span>clonable</span> is set, then append
        "<code data-x=""> shadowrootclonable=&quot;&quot;</code>".</p></li>
 
-       <li><p>If <var>current node</var>'s <span data-x="element-custom-element-registry">custom
-       element registry</span> is not <var>shadow</var>'s <span
-       data-x="shadow-root-custom-element-registry">custom element registry</span>, then append
-       "<code data-x=""> shadowrootcustomelementregistry=&quot;&quot;</code>".</p></li>
+       <li>
+        <p>Let <var>shouldAppendRegistryAttribute</var> be the result of running these steps:</p>
+
+        <ol>
+         <li><p>Let <var>documentRegistry</var> be <var>shadow</var>'s <span>node document</span>'s
+         <span data-x="document-custom-element-registry">custom element registry</span>.</p></li>
+
+         <li><p>Let <var>shadowRegistry</var> be <var>shadow</var>'s <span
+         data-x="shadow-root-custom-element-registry">custom element registry</span>.</p></li>
+
+         <li><p>If <var>documentRegistry</var> is null and <var>shadowRegistry</var> is null, then
+         return false.</p></li>
+
+         <li><p>If <var>documentRegistry</var> <span>is a global custom element registry</span> and
+         <var>shadowRegistry</var> <span>is a global custom element registry</span>, then return
+         false.</p></li>
+
+         <li><p>Return true.</p></li>
+        </ol>
+       </li>
+
+       <li><p>If <var>shouldAppendRegistryAttribute</var> is true, then append "<code
+       data-x=""> shadowrootcustomelementregistry=&quot;&quot;</code>".</p></li>
 
        <li><p>Append "<code data-x="">&gt;</code>".</p></li>