diff --git a/source b/source
index 619d28c597e..5bd5a7c7e3b 100644
--- a/source
+++ b/source
@@ -56744,6 +56744,10 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
   </ol>
 
+  <p class="warning">There is no well defined algorithm on selecting <var>control</var> if Shadow
+  DOM is used in a <code>dialog</code> element. See issue
+  <a href="https://github.com/whatwg/html/issues/2393">#2393</a>.</p>
+
   <p>If at any time a <code>dialog</code> element is <span data-x="node is removed from a
   document">removed from a <code>Document</code></span>, then if that <code>dialog</code> is in that
   <code>Document</code>'s <span>pending dialog stack</span>, the following steps must be run:</p>