diff --git a/source b/source
index c127ef318d6..42db70e37d2 100644
--- a/source
+++ b/source
@@ -3053,7 +3053,10 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x="event-keydown" data-x-href="https://w3c.github.io/uievents/#event-type-keydown"><code>keydown</code></dfn> event</li>
      <li><dfn data-x="event-keypress" data-x-href="https://w3c.github.io/uievents/#event-type-keypress"><code>keypress</code></dfn> event</li>
      <li><dfn data-x="event-keyup" data-x-href="https://w3c.github.io/uievents/#event-type-keyup"><code>keyup</code></dfn> event</li>
-    </ul>
+
+     <li>The <dfn data-x="event-target" data-x-href="https://w3c.github.io/uievents/#event-target">event target</dfn> concept</li>
+     <li>The <dfn data-x="topmost-event-target" data-x-href="https://w3c.github.io/uievents/#topmost-event-target">topmost event target</dfn> concept</li>
+  </ul>
 
     <p>The following features are defined in <cite>Touch Events</cite>: <ref spec=TOUCH></p>
 
@@ -55125,6 +55128,8 @@ MIT Room 32-G524
    apply">does not apply</span> to this element, throw an
    <span>"<code>InvalidStateError</code>"</span> <code>DOMException</code>.</p></li>
 
+   <li><p>If this element is <code>inert</code>, return.</p></li>
+
    <li><p><span>Set the selection range</span> with <var>start</var>, <var>end</var>, and
    <var>direction</var>.</p></li>
   </ol>
@@ -72506,26 +72511,63 @@ END:VCARD</pre>
 
   <h3>Inert subtrees</h3>
 
-  <p class="note">This section <strong>does not</strong> define or create any content attribute
-  named "inert". This section merely defines an abstract <em>concept</em> of
-  <span data-x="inert">inertness</span>.</p>
+  <p class="note">See also <code data-x="attr-inert">inert</code> for an explanation of
+  the attribute of the same name.</p>
 
   <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
-  <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
-  from the <code>body</code> over to the <span>inert</span> paragraph and clicks on the paragraph,
-  no <code data-x="event-mouseover">mouseover</code> event would be fired, and the <code
-  data-x="event-mousemove">mousemove</code> and <code data-x="event-click">click</code> events would
-  be fired on the <code>body</code> element rather than the paragraph.</p>
+  is <span>inert</span>, then:</p>
+
+  <ul>
+   <li>
+    <p>If an event would be fired as a result of user input, such that the
+    <span>inert</span> element would be the <span data-x="topmost-event-target">topmost event
+    target</span>, an alternative <span data-x="event-target">event target</span> must be found as
+    follows:</p>
+    <ol>
+     <li><p>Let <var>originalElement</var> be the <span data-x="topmost-event-target">topmost event
+     target</span> for the event.</p></li>
+     <li><p>Let <var>candidate</var> be the same element.</p></li>
+     <li><p>Let <var>ineligible</var> be an empty list.</p></li>
+     <li>
+      <p>While <var>candidate</var> is <span>inert</span>, or <var>candidate</var> is not an an
+      ancestor of <var>originalElement</var> in the <span>flat tree</span>:</p>
+      <ol>
+       <li><p>Add <var>candidate</var> to <var>ineligible</var>.</p></li>
+       <li><p>Set <var>candidate</var> to the element which would be the <span
+       data-x="topmost-event-target">topmost event target</span> if all the elements in
+       <var>ineligible</var> were excluded from consideration, or null if no such element
+       exists.</p></li>
+      </ol>
+     </li>
+     <li><p>If <var>candidate</var> is not null, let <var>candidate</var> be the new <span
+     data-x="event-target">event target</span>.</p></li>
+     <li><p>Otherwise, do not fire the event.</p></li>
+    </ol>
+   </li>
+
+   <li><p>The user agent may ignore the node for the purposes of text search user interfaces
+   (commonly known as "find in page"); and</p></li>
+
+   <li><p>The user agent may prevent the user from selecting text in that node.</p></li>
+  </ul>
+
+  <p>User agents may allow the user to override the restrictions on search and text selection,
+  however.</p>
+
+  <p class="example">Consider a page that consists of just a single <span>inert</span>
+  <code>button</code> positioned in the middle of a <code>body</code>. If a user clicks on the
+  button, the <code data-x="event-click">click</code> event would cause no event listeners on
+  <code>button</code> to be run, and the event would instead be targeted at the <code>body</code>
+  element.</p>
 
   <p class="note">When a node is inert, it generally cannot be focused. Inert nodes that are <span
   data-x="concept-command">commands</span> will also get disabled.</p>
 
+  <p>User agents should show the default cursor when a user indicates an element in an inert subtree
+  using a pointing device.</p>
+
+  <p>By default, an element is not <span>inert</span>.</p>
+
   <p>While a <span>browsing context container</span> is marked as <span>inert</span>, its
   <span>nested browsing context</span>'s <span>active document</span>, and all nodes in that
   <code>Document</code>, must be marked as <span>inert</span>.</p>
@@ -72548,7 +72590,27 @@ END:VCARD</pre>
   data-x="top-layer-add">adding</span> the <code>dialog</code> element to its <span>node
   document</span>'s <span>top layer</span>.</p>
 
+  <h4>The <dfn data-x="attr-inert"><code>inert</code></dfn> attribute</h4>
+
+  <p>The <code data-x="attr-inert">inert</code> attribute is a <span>boolean attribute</span> that
+  indicates, by its presence, that the element and all its shadow-including descendants is to be
+  made <span>inert</span>.</p>
 
+  <p w-nodev>While an element has an <code data-x="attr-inert">inert</code> attribute, the user
+  agent must mark that element and all its <span>flat tree</span> descendants as
+  <span>inert</span>.</p>
+
+  <p class="note">By default, there is no persistent visual indication of a subtree being inert.
+  Authors are encouraged to clearly mark what parts of their document are active and which are
+  inert, to avoid user confusion.  In particular, it is worth remembering that not all users can see
+  all parts of a page at once; for example, users of screen readers, users on small devices or with
+  magnifiers, and even users just using particularly small windows might not be able to see the
+  active part of a page and might get frustrated if inert sections are not obviously inert. For
+  individual controls, the <code data-x="attr-fe-disabled">disabled</code> attribute is probably
+  more appropriate.</p>
+
+  <p>The <dfn data-x="dom-inert"><code>inert</code></dfn> IDL attribute must <span>reflect</span>
+  the content attribute of the same name.</p>
 
   <h3>Tracking user activation</h3>