diff --git a/source b/source
index 8639eb9e205..ab7d7657bb0 100644
--- a/source
+++ b/source
@@ -4129,8 +4129,13 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://drafts.csswg.org/css-contain/#content-visibility">'content-visibility'</dfn> property</li>
      <li><dfn data-x-href="https://drafts.csswg.org/css-contain/#propdef-content-visibility" data-x="content-visibility-auto">'auto'</dfn> value for <span>'content-visibility'</span></li>
     </ul>
-   </dd>
 
+    <p>The following terms are defined in <cite>CSS Anchor Positioning</cite>: <ref>CSSANCHOR</ref></p>
+
+    <ul class="brief">
+     <li><dfn data-x-href="https://drafts.csswg.org/css-anchor-position-1/#implicit-anchor-element">implicit anchor element</dfn></li>
+    </ul>
+   </dd>
 
    <dt>Intersection Observer</dt>
 
@@ -11771,6 +11776,9 @@ interface <dfn interface>HTMLElement</dfn> : <span>Element</span> {
   undefined <span data-x="dom-hidePopover">hidePopover</span>();
   boolean <span data-x="dom-togglePopover">togglePopover</span>(optional boolean force);
   [<span>CEReactions</span>] attribute DOMString? <span data-x="dom-popover">popover</span>;
+
+  // The anchor attribute
+  [<span>CEReactions</span>] attribute Element? <span data-x="dom-anchorElement">anchorElement</span>;
 };
 
 <span>HTMLElement</span> includes <span>GlobalEventHandlers</span>;
@@ -13005,6 +13013,7 @@ https://software.hixie.ch/utilities/js/live-dom-viewer/?%3C%21DOCTYPE%20HTML%3E%
 
   <ul class="brief">
    <li><code data-x="attr-accesskey">accesskey</code></li>
+   <li><code data-x="attr-anchor">anchor</code></li>
    <li><code data-x="attr-autocapitalize">autocapitalize</code></li>
    <li><code data-x="attr-fe-autofocus">autofocus</code></li>
    <li><code data-x="attr-contenteditable">contenteditable</code></li>
@@ -14011,6 +14020,44 @@ background: transparent">blue&lt;/span>.&lt;/p></code></pre>
   </div>
 
 
+  <h5>The <code data-x="attr-anchor">anchor</code> attribute</h5>
+
+  <p>The <dfn element-attr for="html-global"><code data-x="attr-anchor">anchor</code></dfn>
+  attribute allows authors to set a <span data-x="referenced">reference</span> to the <span>implicit
+  anchor element</span>.  If an <span data-x="HTML elements">HTML element</span> has an <code
+  data-x="attr-anchor">anchor</code> attribute specified, and there is an element in the
+  <span>tree</span> whose <span data-x="concept-id">ID</span> is equal to the value of the <code
+  data-x="attr-anchor">anchor</code> attribute, then the first such element in <span>tree
+  order</span> is the <span>implicit anchor element</span> of the element with the <code
+  data-x="attr-anchor">anchor</code> attribute. <ref>CSSANCHOR</ref></p>
+
+  <p>The <dfn data-x="dom-anchorElement"><code>anchorElement</code></dfn> IDL attribute must
+  <span>reflect</span> the <code data-x="attr-anchor">anchor</code> attribute.
+
+  <div class="example">
+   <p>The following example shows how to use CSS anchor positioning by using the <code
+   data-x="attr-anchor">anchor</code> attribute to set the implicit anchor element:</p>
+
+   <pre><code class="html">&lt;div id="myanchor">myanchor&lt;/div>
+&lt;div id="target" anchor="myanchor">target&lt;/div>
+
+&lt;style>
+#myanchor {
+  margin: 100px;
+  width: 100px;
+  height: 100px;
+}
+#target {
+  position: absolute;
+  left: anchor(left);
+  top: anchor(bottom);
+  width: 100px;
+  height: 100px;
+}
+&lt;/style></code></pre>
+  </div>
+
+
 
   <h5><dfn>Embedding custom non-visible data</dfn> with the <code data-x="attr-data-*">data-*</code> attributes</h5>
 
@@ -85284,13 +85331,17 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
    <p>The <span>topmost popover ancestor</span> algorithm will return the topmost (latest in the
    <span>showing auto popover list</span>) ancestor popover for the provided popover or top layer
    element. Popovers can be related to each other in several ways, creating a tree of popovers.
-   There are two paths through which one popover (call it the "child" popover) can have a topmost
+   There are three paths through which one popover (call it the "child" popover) can have a topmost
    ancestor popover (call it the "parent" popover):</p>
 
    <ol>
     <li><p>The popovers are nested within each other in the node tree. In this case, the descendant
     popover is the "child" and its topmost ancestor popover is the "parent".</p></li>
 
+    <li><p>A popover has the <code data-x="attr-anchor">anchor</code> attribute set and it points
+    to an element whose ancestor is also a popover. In this case, the popover with the <code
+    data-x="attr-anchor">anchor</code> attribute is the "child".</p></li>
+
     <li><p>An invoking element (e.g., a <code>button</code>) has a <code
     data-x="attr-popovertarget">popovertarget</code> attribute pointing to a popover. In this case,
     the popover is the "child", and the popover subtree the invoking element is in is the
@@ -85387,6 +85438,9 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
    <li><p>Run <var>checkAncestor</var> given <var>newPopoverOrTopLayerElement</var>'s parent node
    within the <span>flat tree</span>.</p></li>
 
+   <li><p>Run <var>checkAncestor</var> given the result of running <span>get an attribute by
+   name</span> given <var>newPopover</var> and <code data-x="attr-anchor">anchor</code>.</p></li>
+
    <li><p>Run <var>checkAncestor</var> given <var>invoker</var>.</p></li>
 
    <li><p>Return <var>topmostPopoverAncestor</var>.</p></li>
@@ -132500,6 +132554,10 @@ dialog:popover-open {
   background-color: Canvas;
 }
 
+[popover][anchor] {
+  inset: auto;
+}
+
 :popover-open::backdrop {
   position: fixed;
   inset: 0;
@@ -139646,6 +139704,11 @@ interface <dfn interface>External</dfn> {
           <code data-x="attr-input-alt">input</code>
      <td> Replacement text for use when images are not available
      <td> <a href="#attribute-text">Text</a>*
+    <tr>
+     <th> <code data-x="">anchor</code>
+     <td> <span data-x="attr-anchor">HTML elements</span>
+     <td> Sets the <span>implicit anchor element</span> of this element. Also influences the popover hierarchy if the specified element also has the <code data-x="attr-popover">popover</code> attribute.
+     <td> <span data-x="concept-id">ID</span>*
     <tr>
      <th> <code data-x="">as</code>
      <td> <code data-x="attr-link-as">link</code>
@@ -142191,6 +142254,9 @@ INSERT INTERFACES HERE
    <dt id="refsCSSALIGN">[CSSALIGN]</dt>
    <dd><cite><a href="https://w3c.github.io/csswg-drafts/css-align/">CSS Box Alignment</a></cite>, E. Etemad, T. Atkins. W3C.</dd>
 
+   <dt id="refsCSSANCHOR">[CSSANCHOR]</dt>
+   <dd><cite><a href="https://drafts.csswg.org/css-anchor-position-1/">CSS Anchor Positioning</a></cite>, T. Atkins, J. Tompkins, I. Kilpatrick. W3C.</dd>
+
    <dt id="refsCSSANIMATIONS">[CSSANIMATIONS]</dt>
    <dd><cite><a href="https://w3c.github.io/csswg-drafts/css-animations/">CSS Animations</a></cite>, D. Jackson, D. Hyatt, C. Marrin, S. Galineau, L. Baron. W3C.</dd>