diff --git a/source b/source
index 278b5617197..b1a6bcaf55b 100644
--- a/source
+++ b/source
@@ -3979,6 +3979,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
       <li><dfn data-x-href="https://drafts.csswg.org/css-sizing/#fit-content-inline-size">fit-content inline size</dfn></li>
       <li><dfn data-x-href="https://drafts.csswg.org/css-sizing-4/#aspect-ratio">'aspect-ratio'</dfn> property</li>
       <li><dfn data-x-href="https://drafts.csswg.org/css-sizing/#intrinsic-size">intrinsic size</dfn></li>
+      <li><dfn data-x-href="https://drafts.csswg.org/css-sizing/#document-responsive-embedded-sizing-flag">responsive embedded sizing flag</dfn></li>
     </ul>
 
     <p>The following features are defined in <cite>CSS Lists and Counters</cite>.
@@ -11497,6 +11498,10 @@ partial interface <dfn id="document" data-lt="">Document</dfn> {
   navigation ID for WebDriver BiDi</dfn>, which is a <span>navigation ID</span> or null, initially
   null.</p>
 
+  <p>Each <code>Document</code> has a
+  <dfn data-x="concept-document-responsive-embedded-sizing-flag">responsive embedded sizing flag</dfn>,
+  which is a boolean, initially false.</p>
+
   <p class="note">As the name indicates, this is used for interfacing with the <cite>WebDriver
   BiDi</cite> specification, which needs to be informed about certain occurrences during the early
   parts of the <code>Document</code>'s lifecycle, in a way that ties them to the original
@@ -17901,6 +17906,23 @@ interface <dfn interface>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
     </div>
    </dd>
 
+   <dt><dfn attr-value for="meta/name"><code data-x="meta-responsive-embedded-sizing">responsive-embedded-sizing</code></dfn></dt>
+
+   <dd>
+    <div algorithm>
+    <p>If a <code>meta</code> element <var>element</var> is
+    <span data-x="node is inserted into a document">inserted into the document</span>,
+    the user agent must run the following algorithm:</p>
+
+    <ol>
+     <li><p>If <var>element</var> is not inserted by the parser, then return.</p></li>
+
+     <li><p>Set the <span>responsive embedded sizing flag</span>
+     of the <code>Document</code>.<p></li>
+    </ol>
+    </div>
+   </dd>
+
    <dt><dfn attr-value for="meta/name"><code data-x="meta-theme-color">theme-color</code></dfn></dt>
 
    <dd>