diff --git a/source b/source
index 278b5617197..7efb88cbbb8 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-4/#responsive-iframes">responsively-sized iframes</dfn></li>
     </ul>
 
     <p>The following features are defined in <cite>CSS Lists and Counters</cite>.
@@ -11504,6 +11505,15 @@ partial interface <dfn id="document" data-lt="">Document</dfn> {
   the <span>ongoing navigation</span>. This eventually gets set back to null, after <cite>WebDriver
   BiDi</cite> considers the loading process to be finished. <ref>BIDI</ref></p>
 
+  <p>Each <code>Document</code> has a boolean
+  <dfn expoprt data-x="concept-document-responsive-embedded-sizing">responsive embedded
+  sizing</dfn>, initially false.</p>
+
+  <p class="note">This is set by <code>meta</code> elements whose
+  <code data-x="attr-meta-name">name</code> attribute is
+  <code data-x="meta-responsive-embedded-sizing">responsive-embedded-sizing</code>,
+  and used by <span>responsively-sized iframes</span>.</p>
+
   <p>Each <code>Document</code> has an <dfn data-x="concept-document-about-base-url">about base
   URL</dfn>, which is a <span>URL</span> or null, initially null.</p>
 
@@ -17901,6 +17911,36 @@ 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 any <code>meta</code> element <var>element</var> is <span data-x="node is inserted into a
+    document">inserted into the document</span>, or has its <code
+    data-x="attr-meta-name">name</code> or <code data-x="attr-meta-content">content</code>
+    attributes changed, user agents must run the following algorithm:</p>
+
+    <ol>
+     <li><p>If <var>element</var> is not <span>in a document tree</span>, then return.</p></li>
+
+     <li><p>If <var>element</var> does not have a <code data-x="attr-meta-name">name</code>
+     attribute whose value is an <span>ASCII case-insensitive</span> match for "<code
+     data-x="meta-responsive-embedded-sizing">responsive-embedded-sizing</code>",
+     then return.</p></li>
+
+     <li><p>If <var>element</var> has a <code data-x="attr-meta-content">content</code>
+     attribute and that attribute's value is not the empty string, then return.</p></li>
+
+     <li><p>If <span>the body element</span> of the <code>Document</code> is not null,
+     then return.</p></li>
+
+     <li><p>Set the <code>Document</code>'s
+     <span data-x="concept-document-responsive-embedded-sizing">responsive embedded sizing</span>
+     to true.<p></li>
+    </ol>
+    </div>
+   </dd>
+
    <dt><dfn attr-value for="meta/name"><code data-x="meta-theme-color">theme-color</code></dfn></dt>
 
    <dd>