Define the foundations of find-in-page

This serves as helpful structure for work such as #3539, or potentially
integrating with https://github.com/WICG/scroll-to-text-fragment or
https://github.com/WICG/display-locking.

source

@@ -3075,9 +3075,12 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x="dom-outerHTML" data-x-href="https://w3c.github.io/DOM-Parsing/#dom-element-outerhtml"><code>outerHTML</code></dfn></li>
     </ul>
 
-    <p>The <dfn
-    data-x-href="https://w3c.github.io/selection-api/#selection-interface"><code>Selection</code></dfn>
-    interface is defined in the <cite>Selection API</cite> specification. <ref spec=SELECTION></p>
+    <p>The following features are defined in <cite>Selection API</cite>: <ref spec=SELECTION></p>
+
+    <ul class="brief">
+     <li><dfn data-x="document-selection" data-x-href="https://w3c.github.io/selection-api/#dfn-selection">selection</dfn></li>
+     <li><dfn data-x-href="https://w3c.github.io/selection-api/#selection-interface"><code>Selection</code></dfn></li>
+    </ul>
 
     <p class="note">User agents are encouraged to implement the features described in
     <cite>execCommand</cite>. <ref spec=EXECCOMMAND></p>
@@ -71836,10 +71839,9 @@ END:VCARD</pre>
 
   <p>A node (in particular elements and text nodes) can be marked as <dfn>inert</dfn>. When a node
   is <span>inert</span>, then the user agent must act as if the node was absent for the purposes of
-  targeting user interaction events, may ignore the node for the purposes of text search user
-  interfaces (commonly known as "find in page"), and may prevent the user from selecting text in
-  that node. User agents should allow the user to override the restrictions on search and text
-  selection, however.</p>
+  targeting user interaction events, may ignore the node for the purposes of
+  <span>find-in-page</span>, and may prevent the user from selecting text in that node. User agents
+  should allow the user to override the restrictions on search and text selection, however.</p>
 
   <p class="example">For example, consider a page that consists of just a single <span>inert</span>
   paragraph positioned in the middle of a <code>body</code>. If a user moves their pointing device
@@ -74583,6 +74585,47 @@ body { display:none }
 
   </div>
 
+  <h3>Find-in-page</h3>
+
+  <h4>Introduction</h4>
+
+  <p>This section defines <dfn>find-in-page</dfn> — a common user-agent mechanism which allows users
+  to search through the contents of the page for particular information.</p>
+
+  <p>Access to <span>find-in-page</span> feature is provided via a <dfn>find-in-page
+  interface</dfn>. This is a user-agent provided user interface, which allows the user to specify
+  input and the parameters of the search. This interface can appear as a result of a shortcut or a
+  menu selection.</p>
+
+  <p>A combination of text input and settings in the <span>find-in-page interface</span> represents
+  the user <dfn data-x="fip-query">query</dfn>. This typically includes the text that the user wants
+  to search for, as well as optional settings (e.g., the ability to restrict the search to whole
+  words only).</p>
+
+  <p>The user-agent processes page contents for a given <span data-x="fip-query">query</span>, and
+  identifies zero or more <dfn data-x="fip-matches">matches</dfn>, which are content ranges that
+  satisfy the user <span data-x="fip-query">query</span>.</p>
+
+  <p>One of the <span data-x="fip-matches">matches</span> is identified to the user as the <dfn
+  data-x="fip-active-match">active match</dfn>. It is highlighted and scrolled into view. The user
+  can navigate through the <span data-x="fip-matches">matches</span> by advancing the <span
+  data-x="fip-active-match">active match</span> using the <span>find-in-page interface</span>.</p>
+
+  <p class="XXX"><a href="https://github.com/whatwg/html/issues/3539">Issue #3539</a> tracks
+  standardizing how <span>find-in-page</span> underlies the currently-unspecified <code
+  data-x="">window.find()</code> API.</p>
+
+  <h4>Interaction with selection</h4>
+
+  <p>The find-in-page process is invoked in the context of a document, and may have an effect on
+  the <span data-x="document-selection">selection</span> of that document. Specifically, the range
+  that defines the <span data-x="fip-active-match">active match</span> can dictate the current
+  selection. These selection updates, however, can happen at different times during the
+  find-in-page process (e.g. upon the <span>find-in-page interface</span> dismissal or upon a change
+  in the <span data-x="fip-active-match">active match</span> range).
+
+  <!-- TODO: Define scroll and highlight timing, as well as algorithms (what is and isn't found) -->
+
   <h3 split-filename="dnd" id="dnd"><dfn>Drag and drop</dfn></h3>
 
 <!-- v2: ideas for drag and drop: