diff --git a/source b/source
index 2f6d690e48d..b0b73bfe9c8 100644
--- a/source
+++ b/source
@@ -3359,6 +3359,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <ul class="brief">
      <li>The <dfn data-x-href="https://w3c.github.io/uievents/#mouseevent"><code>MouseEvent</code></dfn> interface</li>
      <li>The <code>MouseEvent</code> interface's <dfn data-x="dom-MouseEvent-relatedTarget" data-x-href="https://w3c.github.io/uievents/#dom-mouseevent-relatedtarget"><code>relatedTarget</code></dfn> attribute</li>
+     <li>The <code>MouseEvent</code> interface's <dfn data-x="dom-MouseEvent-button" data-x-href="https://w3c.github.io/uievents/#dom-mouseevent-button"><code>button</code></dfn> attribute</li>
      <li><dfn data-x-href="https://w3c.github.io/uievents/#dictdef-mouseeventinit"><code>MouseEventInit</code></dfn> dictionary type</li>
 
      <li>The <dfn data-x-href="https://w3c.github.io/uievents/#focusevent"><code>FocusEvent</code></dfn> interface</li>
@@ -62433,6 +62434,9 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
   <var>event</var>:</p>
 
   <ol>
+   <li><p>If <var>event</var>'s <code data-x="dom-MouseEvent-button">button</code> is not 0, then
+   return.</p></li>
+
    <li><p>Run <span>light dismiss open popovers</span> with <var>event</var>.</p></li>
 
    <li><p>Run <span>light dismiss open dialogs</span> with <var>event</var>.</p></li>