diff --git a/source b/source
index be9f621d529..5f2792518e1 100644
--- a/source
+++ b/source
@@ -3587,6 +3587,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
       <li>The <dfn data-x-href="https://drafts.csswg.org/css-logical/#propdef-inline-size">'inline-size'</dfn> property</li>
       <li>The <dfn data-x-href="https://drafts.csswg.org/css-logical/#propdef-inset-block-start">'inset-block-start'</dfn> property</li>
       <li>The <dfn data-x-href="https://drafts.csswg.org/css-logical/#propdef-inset-block-end">'inset-block-end'</dfn> property</li>
+      <li>The <dfn data-x-href="https://drafts.csswg.org/css-logical/#propdef-inset">'inset'</dfn> property</li>
     </ul>
 
     <p>The following terms and features are defined in <cite>CSS Color</cite>: <ref
@@ -3626,7 +3627,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     spec=CSSBG></p>
 
     <ul class="brief">
-      <li>The <dfn data-x-href="https://drafts.csswg.org/css-backgrounds/#propdef-background-color">'background-color'</dfn>,
+      <li>The <dfn data-x-href="https://drafts.csswg.org/css-backgrounds/#propdef-background">'background'</dfn>,
+              <dfn data-x-href="https://drafts.csswg.org/css-backgrounds/#propdef-background-color">'background-color'</dfn>,
               <dfn data-x-href="https://drafts.csswg.org/css-backgrounds/#propdef-background-image">'background-image'</dfn>,
               <dfn data-x-href="https://drafts.csswg.org/css-backgrounds/#propdef-background-repeat">'background-repeat'</dfn>,
               <dfn data-x-href="https://drafts.csswg.org/css-backgrounds/#propdef-background-attachment">'background-attachment'</dfn>,
@@ -3795,6 +3797,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
       <li><dfn data-x-href="https://drafts.csswg.org/css-position/#absolute-position">absolutely-positioned</dfn></li>
       <li>The <dfn data-x-href="https://drafts.csswg.org/css-position/#position-property">'position'</dfn> property and its
               <dfn data-x-href="https://drafts.csswg.org/css-position/#valdef-position-static">'static'</dfn> value</li>
+      <li>The <dfn data-x-href="https://drafts.csswg.org/css-position-4/#overlay">'overlay'</dfn> property</li>
+      <li>The <dfn data-x-href="https://drafts.csswg.org/css-position-4/#backdrop">::backdrop</dfn> pseudo-element</li>
       <li>The <dfn data-x-href="https://drafts.csswg.org/css-position-4/#document-top-layer">top layer</dfn>
               (an <span data-x="set">ordered set</span>)</li>
       <li><dfn data-x-href="https://drafts.csswg.org/css-position-4/#add-an-element-to-the-top-layer">add an element to the top layer</dfn></li>
@@ -3956,6 +3960,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      pseudo-class</li>
      <li><dfn data-x-href="https://drafts.csswg.org/selectors/#indicate-focus">indicate focus</dfn></li>
      <li><dfn data-x-href="https://drafts.csswg.org/selectors/#pseudo-element">pseudo-element</dfn></li>
+     <li><dfn data-x-href="https://drafts.csswg.org/selectors/#originating-element">originating element</dfn></li>
     </ul>
 
     <p>The following features are defined in <cite>CSS Values and Units</cite>: <ref
@@ -124170,16 +124175,6 @@ dialog {
   background-color: Canvas;
   color: CanvasText;
 }
-dialog:modal {
-  position: fixed;
-  overflow: auto;
-  inset-block: 0;
-  max-width: calc(100% - 6px - 2em);
-  max-height: calc(100% - 6px - 2em);
-}
-dialog::backdrop {
-  background: rgba(0,0,0,0.1);
-}
 
 [popover]:not(:popover-open):not(dialog[open]) {
   display:none;
@@ -124202,13 +124197,6 @@ dialog:popover-open {
   background-color: Canvas;
 }
 
-:popover-open::backdrop {
-  position: fixed;
-  inset: 0;
-  pointer-events: none !important;
-  background-color: transparent;
-}
-
 slot {
   display: contents;
 }</code></pre>
@@ -124252,6 +124240,42 @@ form { margin-block-end: 1em; }</code></pre>
   <span>'text-align'</span> property set to 'justify' in a <span data-x="presentational
   hints">presentational hint</span>, and to <span>align descendants</span> to the left.</p>
 
+  <p>The <code>dialog</code> element is expected to act as if it had a user-agent-level style sheet
+  rule setting the following properties when it is in the <span>top layer</span>:</p>
+
+  <ul>
+   <li><span>'position'</span> property to 'fixed'</li>
+   <li><span>'overflow'</span> property to 'auto'</li>
+   <li><span>'inset-block-start'</span> property to '0'</li>
+   <li><span>'inset-block-end'</span> property to '0'</li>
+   <li><span>'max-width'</span> property to 'calc(100% - 6px - 2em)'</li>
+   <li><span>'max-height'</span> property to 'calc(100% - 6px - 2em)'</li>
+  </ul>
+
+  <p>The <code>::backdrop</code> <span>pseudo-element</span> is expected to act as if it had a
+  user-agent-level style sheet rule setting the following properties when its <span>originating
+  element</span> is a <code>dialog</code> in the <span>top layer</span>:</p>
+
+  <ul>
+   <li><span>'position'</span> property to 'fixed'</li>
+   <li><span>'inset'</span> to '0'</li>
+   <li><span>'background'</span> to 'rgba(0,0,0,0.1)'</li>
+  </ul>
+
+  <p>The <code>::backdrop</code> <span>pseudo-element</span> is expected to act as if it had a
+  user-agent-level style sheet rule setting the following properties when its <span>originating
+  element</span> is in the <span data-x="popover-showing-state">popover showing state</span> or
+  while the <span>originating element</span> is still in the <span>top layer</span> immediately
+  following its transition from the <span data-x="popover-showing-state">popover showing
+  state</span> to the <span data-x="popover-hidden-state">popover hidden state</span>:</p>
+
+  <ul>
+   <li><span>'position'</span> property to 'fixed'</li>
+   <li><span>'inset'</span> property to '0'</li>
+   <li><span>'pointer-events'</span> property to 'none !important'</li>
+   <li><span>'background-color'</span> property to 'transparent'</li>
+  </ul>
+
   <h4>Phrasing content</h4>
 
   <pre><code class="css">@namespace "http://www.w3.org/1999/xhtml";