Add hidden=until-found HTML attribute and beforematch event

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@@ -74765,6 +74765,47 @@ END:VCARD</pre>
 
   </div>
 
+  <h4>The <code data-x="attr-hidden-until-found">hidden=until-found</code> attribute</h4>
+
+  <p>When HTML elements have the <code>until-found</code> value set for the <code>hidden</code>
+  attribute, the content inside of them is accessible to <span>find-in-page</span> and <span
+  data-x="navigate-fragid">fragment navigation</span>. When these features attempt to scroll to a
+  target element which is inside a <code>hidden=until-found</code> subtree, the user agent must
+  remove the <code>hidden</code> attribute in order to reveal the content before scrolling to
+  it. In addition to removing the <code>hidden</code> attribute, an event named
+  <span>beforematch</span> is also fired on the <code>hidden</code> element before the
+  <code>hidden</code> attribute is removed.</p>
+
+  <p>Instead of applying the <code>display: none</code> style like the regular hidden attribute,
+  <code>hidden=until-found</code> uses applies <code>content-visibility: hidden</code> in order to
+  improve the performance of find-in-page in these sections.</p>
+
+  <p>The <dfn>ancestor hidden=until-found revealing algorithm</dfn> is to run the following steps on
+  <var>currentNode</var>:</p>
+
+  <ol>
+   <li>
+    <p>While <var>currentNode</var> has a parent node within the <span>flat tree</span>:</p>
+
+    <ol>
+     <li>
+      <p>If <var>currentNode</var> has the <code>hidden=until-found</code> attribute, then run
+      the following steps:</p>
+
+      <ol>
+       <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
+       data-x="event-beforematch">beforematch</code> at <var>currentNode</var>.</p></li>
+
+       <li><p>Remove the <code>hidden</code> attribute from <var>currentNode</var>.</p></li>
+      </ol>
+     </li>
+
+     <li><p>Set <var>currentNode</var> to the parent node of <var>currentNode</var> within the
+     <span>flat tree</span>.</p></li>
+    </ol>
+   </li>
+  </ol>
+
   <h3>Page visibility</h3>
 
   <p>A <span>top-level browsing context</span> has a <dfn>system visibility state</dfn>,
@@ -77759,16 +77800,19 @@ body { display:none }
   standardizing how <span>find-in-page</span> underlies the currently-unspecified <code
   data-x="">window.find()</code> API.</p>
 
-  <h4>Interaction with <code>details</code></h4>
+  <h4>Interaction with <code>details</code> and <code>hidden=until-found</code></h4>
 
   <p>When find-in-page begins searching for matches, all <code>details</code> elements in the page
   which do not have their <code data-x="attr-details-open">open</code> attribute set should have
   the <span data-x="skips its contents">skipped contents</span> of their second slot become
   accessible, without modifying the <code data-x="attr-details-open">open</code> attribute, in
-  order to make find-in-page able to search through it. After find-in-page finishes searching for
-  matches, those <code>details</code> elements should have their contents become skipped again.
-  This entire process must happen synchronously (and so is not observable to users or to author
-  code). <ref spec=CSSCONTAIN></p>
+  order to make find-in-page able to search through it. Similarly, all HTML elements with the
+  <span>hidden=until-found</span> attribute should have their <span data-x="skips its
+  contents">skipped contents</span> become accessible without modifying the <span>hidden</span>
+  attribute in order to make find-in-page able to search through them. After find-in-page finishes
+  searching for matches, the <code>details</code> elements and the <span>hidden=until-found</span>
+  elements should have their contents become skipped again. This entire process must happen
+  synchronously (and so is not observable to users or to author code). <ref spec=CSSCONTAIN></p>
 
   <p>When find-in-page chooses a new <span data-x="fip-active-match">active match</span>, perform
   the following steps:</p>
@@ -77778,8 +77822,17 @@ body { display:none }
    match</span>.</p></li>
 
    <li><p><span>Queue a global task</span> on the <span>user interaction task source</span> given
-   <var>node</var>'s <span>relevant global object</span> to run the <span>ancestor details
-   revealing algorithm</span> on <var>node</var>.</p></li>
+   <var>node</var>'s <span>relevant global object</span> to run the following steps:</p></li>
+
+   <ol>
+    <li><p>Run the <span>ancestor details revealing algorithm</span> on <var>node</var>.</p></li>
+
+    <li><p>Run the <span>ancestor hidden-until-found revealing algorithm</span> on
+    <var>node</var>.</p></li>
+
+    <li><p><span data-x="scroll an element into view">Scroll <var>node</var> into
+    view</span>.</p></li>
+   </ol>
   </ol>
 
   <p class="warning">
@@ -89189,6 +89242,9 @@ new PaymentRequest(&hellip;); // Allowed to use
 
      <li><p>Run the <span>ancestor details revealing algorithm</span> on <var>target</var>.</p></li>
 
+     <li><p>Run the <span>ancestor hidden-until-found revealing algorithm</span> on
+     <var>target</var>.</p></li>
+
      <li><p><span data-x="scroll an element into view">Scroll <var>target</var> into view</span>,
      with <var>behavior</var> set to "auto", <var>block</var> set to "start", and <var>inline</var>
      set to "nearest". <ref spec=CSSOMVIEW></p></li>
@@ -126244,6 +126300,13 @@ INSERT INTERFACES HERE
      <td> <code>Window</code>
      <td> Fired at the <code>Window</code> before printing
 
+    <tr> <!-- beforematch -->
+     <td> <dfn event for="HTMLElement"><code data-x="event-beforematch">beforematch</code></dfn>
+     <td> <code>Event</code>
+     <td> Elements
+     <td> Fired on elements with the <span>hidden=until-found</span> attribute before they are
+     revealed.
+
     <tr> <!-- beforeunload -->
      <td> <dfn event for="Window"><code data-x="event-beforeunload">beforeunload</code></dfn>
      <td> <code>BeforeUnloadEvent</code>