Add hidden=until-found HTML attribute and beforematch event

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@@ -74663,12 +74663,11 @@ END:VCARD</pre>
   <h3>The <code data-x="attr-hidden">hidden</code> attribute</h3>
 
   <p>All <span>HTML elements</span> may have the <dfn element-attr for="html-global"><code
-  data-x="attr-hidden">hidden</code></dfn> content attribute set. The <code
-  data-x="attr-hidden">hidden</code> attribute is a <span>boolean attribute</span>. When specified
-  on an element, it indicates that the element is not yet, or is no longer, directly relevant to the
-  page's current state, or that it is being used to declare content to be reused by other parts of
-  the page as opposed to being directly accessed by the user. <span w-nodev>User agents should not
-  render elements that have the <code data-x="attr-hidden">hidden</code> attribute specified. This
+  data-x="attr-hidden">hidden</code></dfn> content attribute set. When specified on an element, it
+  indicates that the element is not yet, or is no longer, directly relevant to the page's current
+  state, or that it is being used to declare content to be reused by other parts of the page as
+  opposed to being directly accessed by the user. <span w-nodev>User agents should not render
+  elements that have the <code data-x="attr-hidden">hidden</code> attribute specified. This
   requirement may be implemented indirectly through the style layer. For example, an HTML+CSS user
   agent could implement these requirements <a href="#hiddenCSS">using the rules suggested in the
   Rendering section</a>.</span></p>
@@ -74755,6 +74754,53 @@ END:VCARD</pre>
   <p>The <dfn attribute for="HTMLElement"><code data-x="dom-hidden">hidden</code></dfn> IDL
   attribute must <span>reflect</span> the content attribute of the same name.</p>
 
+  <p>When an element has the value <dfn><code
+  data-x="attr-hidden-until-found">"until-found"</code></dfn> for the hidden attribute, the content
+  inside the element should be 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 which is in the element's subtree, the user agent is expected to remove the <code
+  data-x="attr-hidden">hidden</code> attribute in order to reveal the content before scrolling to
+  it. In addition to removing the <code data-x="attr-hidden">hidden</code> attribute, an event named
+  <span data-x="event-beforematch">beforematch</span> is also fired on the <code
+  data-x="attr-hidden">hidden</code> element before the <code data-x="attr-hidden">hidden</code>
+  attribute is removed.</p>
+
+  <p>User agents are expected to use content-visibility:hidden instead of display:none when the
+  hidden attribute is <code data-x="attr-hidden-until-found">"until-found"</code>, as specified in
+  the <a href="#hiddenCSS">Rendering section</a>.</p>
+
+  <p class="warning">Since hidden=until-found uses content-visibility:hidden instead of
+  display:none, the hidden element is expected to be display:block and will have a <a
+  href="https://drafts.csswg.org/css2/#visibility">generated box</a> unlike display:none.</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
+      data-x="attr-hidden-until-found">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 data-x="attr-hidden">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>
+
   </div>
 
   <h3>Page visibility</h3>
@@ -77751,16 +77797,21 @@ 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
+  data-x="attr-hidden-until-found">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 <code
+  data-x="attr-hidden-until-found">hidden=until-found</code> attribute should have their <span
+  data-x="skips its contents">skipped contents</span> become accessible without modifying the <span
+  data-x="attr-hidden">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
+  <code data-x="attr-hidden-until-found">hidden=until-found</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>
 
   <p>When find-in-page chooses a new <span data-x="fip-active-match">active match</span>, perform
   the following steps:</p>
@@ -77770,8 +77821,18 @@ 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>
+
+    <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>
+   </li>
   </ol>
 
   <p class="warning">
@@ -89209,6 +89270,9 @@ 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>
@@ -117133,11 +117197,19 @@ console.assert(container.firstChild instanceof SuperP);
 
   <pre><code class="css">@namespace url(http://www.w3.org/1999/xhtml);
 
-<span id="hiddenCSS" data-x="">[hidden]</span>, area, base, basefont, datalist, head, link, meta, noembed,
+area, base, basefont, datalist, head, link, meta, noembed,
 noframes, param, rp, script, style, template, title {
   display: none;
 }
 
+<span id="hiddenCSS" data-x="">[hidden]:not([hidden=until-found i])</span> {
+  display: none;
+}
+
+[hidden=until-found] {
+  content-visibility: hidden;
+}
+
 embed[hidden] { display: inline; height: 0; width: 0; } <!-- because for legacy reasons it still needs to instantiate the plugin -->
 
 input[type=hidden i] { display: none !important; }
@@ -126255,6 +126327,14 @@ 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 <code
+     data-x="attr-hidden-until-found">hidden=until-found</code> attribute before they are
+     revealed.
+
     <tr> <!-- beforeunload -->
      <td> <dfn event for="Window"><code data-x="event-beforeunload">beforeunload</code></dfn>
      <td> <code>BeforeUnloadEvent</code>