diff --git a/source b/source
index 8419eda4468..067049f8c7a 100644
--- a/source
+++ b/source
@@ -3415,6 +3415,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <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>
+     <li><dfn data-x-href="https://dom.spec.whatwg.org/#concept-element-keep-custom-element-registry-null">keep element custom element registry null</dfn></li>
     </ul>
 
     <p>The following features are defined in <cite>UI Events</cite>: <ref>UIEVENTS</ref></p>
@@ -12622,6 +12623,7 @@ interface <dfn interface>HTMLElement</dfn> : <span>Element</span> {
   [<span>CEReactions</span>, <span data-x="xattr-ReflectSetter">ReflectSetter</span>] attribute DOMString <span data-x="dom-writingSuggestions">writingSuggestions</span>;
   [<span>CEReactions</span>, <span data-x="xattr-ReflectSetter">ReflectSetter</span>] attribute DOMString <span data-x="dom-autocapitalize">autocapitalize</span>;
   [<span>CEReactions</span>] attribute boolean <span data-x="dom-autocorrect">autocorrect</span>;
+  [<span>CEReactions</span>, <span data-x="xattr-Reflect">Reflect</span>] attribute boolean <dfn attribute for="HTMLElement" data-x="dom-element-customElementRegistry">customElementRegistry</dfn>;
 
   [<span>CEReactions</span>] attribute [<span>LegacyNullToEmptyString</span>] DOMString <span data-x="dom-innerText">innerText</span>;
   [<span>CEReactions</span>] attribute [<span>LegacyNullToEmptyString</span>] DOMString <span data-x="dom-outerText">outerText</span>;
@@ -13976,6 +13978,7 @@ https://software.hixie.ch/utilities/js/live-dom-viewer/?%3C%21DOCTYPE%20HTML%3E%
    <li><code data-x="attr-autocorrect">autocorrect</code></li>
    <li><code data-x="attr-fe-autofocus">autofocus</code></li>
    <li><code data-x="attr-contenteditable">contenteditable</code></li>
+   <li><code data-x="attr-customelementregistry">customelementregistry</code></li>
    <li><code data-x="attr-dir">dir</code></li>
    <li><code data-x="attr-draggable">draggable</code></li>
    <li><code data-x="attr-enterkeyhint">enterkeyhint</code></li>
@@ -77597,6 +77600,14 @@ console.assert(el instanceof SpiderMan);    // upgraded!</code></pre>
   <code>CustomElementRegistry</code> object, it intentionally cannot be changed any further. This
   simplifies reasoning about code and allows implementations to optimize.</p>
 
+  <h4>The <code data-x="attr-customelementregistry">customelementregistry</code> attribute</h4>
+
+  <p> The
+  <dfn element-attr for="html-global"><code data-x="attr-customelementregistry">customelementregistry</code></dfn>
+  attribute is used to specify whether the parser should be
+  <span data-x="look up a custom element registry">looking up a custom element registry</span>
+  when the parser runs <span>Create an element for the token</span>. The value is boolean.</p>
+
 
   <h4><dfn data-x="custom-element-upgrades">Upgrades</dfn></h4>
 
@@ -139837,8 +139848,15 @@ dictionary <dfn dictionary>StorageEventInit</dfn> : <span>EventInit</span> {
    <li><p>Let <var>is</var> be the value of the "<code data-x="attr-is">is</code>" attribute in
    <var>token</var>, if such an attribute exists; otherwise null.</p></li>
 
-   <li><p>Let <var>registry</var> be the result of <span data-x="look up a custom element
-   registry">looking up a custom element registry</span> given <var>intendedParent</var>.</p></li>
+   <li><p>Let <var>registry</var> be null if
+   <code data-x="attr-customelementregistry">customelementregistry</code> attribute exists;
+   otherwise let <var>registry</var> be the result of
+   <span data-x="look up a custom element registry">looking up a custom element registry</span>
+   given <var>intendedParent</var>.</p></li>
+
+   <li><p>Let <var>keepRegistryNull</var> be true if
+   <code data-x="attr-customelementregistry">customelementregistry</code> attribute exists;
+   otherwise false.
 
    <li><p>Let <var>definition</var> be the result of <span data-x="look up a custom element
    definition">looking up a custom element definition</span> given <var>registry</var>,
@@ -139927,6 +139945,10 @@ dictionary <dfn dictionary>StorageEventInit</dfn> : <span>EventInit</span> {
    with the <code>form</code> element pointed to by the <span><code data-x="">form</code> element
    pointer</span> and set <var>element</var>'s <span>parser inserted flag</span>.</p></li>
 
+   <li><p>If <var>keepRegistryNull</var> is true, set <var>element</var>'s
+   <span>keep element custom element registry null</span>
+   to true.
+
    <li><p>Return <var>element</var>.</p></li>
   </ol>
   </div>
@@ -144648,6 +144670,15 @@ document.body.appendChild(text);
         data-x="escaping a string">escaped as described below</span> in <i>attribute mode</i>,
         followed by a U+0022 QUOTATION MARK character (&quot;).</p>
 
+        <p>If the element does not have a
+        <code data-x="attr-customelementregistry">customelementregistry</code> attribute in its
+        attribute list, and <var>current node</var>'s
+        <span data-x="element-custom-element-registry">custom element registry</span> is not the
+        same as the result of
+        <span data-x="look up a custom element registry">looking up a custom element
+        registry</span> given <var>current node</var>'s parent element, then append
+        "<code data-x=""> customelementregistry=&quot;&quot;</code>".</p>
+
         <p>For each attribute that the element has, append a U+0020 SPACE character, the <span
         data-x="attribute's serialized name">attribute's serialized name as described below</span>, a
         U+003D EQUALS SIGN character (=), a U+0022 QUOTATION MARK character (&quot;), the
@@ -153378,6 +153409,11 @@ interface <dfn interface>External</dfn> {
      <td> "<code data-x="attr-crossorigin-anonymous-keyword">anonymous</code>";
           "<code data-x="attr-crossorigin-use-credentials-keyword">use-credentials</code>";
           the empty string
+    <tr>
+     <th> <code data-x="">customelementregistry</code>
+     <td> <span data-x="attr-class">HTML elements</span>
+     <td> Whether the element should use scoped custom element registry
+     <td> <span>Boolean attribute</span>
     <tr>
      <th> <code data-x="">data</code>
      <td> <code data-x="attr-object-data">object</code>