Clone shadow root's keep custom element registry null

Author: ja-y-son

dom.bs

@@ -4942,6 +4942,10 @@ and an optional <a for=/>document</a> <dfn export for="clone a node"><var>docume
    <li><p>Set <var>copy</var>'s <a for=Element>shadow root</a>'s <a for=ShadowRoot>declarative</a>
    to <var>node</var>'s <a for=Element>shadow root</a>'s <a for=ShadowRoot>declarative</a>.
 
+   <li><p>Set <var>copy</var>'s <a for=Element>shadow root</a>'s
+   <a for=ShadowRoot>keep custom element registry null</a> to <var>node</var>'s
+   <a for=Element>shadow root</a>'s <a for=ShadowRoot>keep custom element registry null</a>.
+
    <li>
     <p>For each <var>child</var> of <var>node</var>'s <a for=Element>shadow root</a>'s
     <a for=tree>children</a>, in <a>tree order</a>: <a>clone a node</a> given <var>child</var> with