Provide a fallback for validation anchor

And allow the fallback to be explicitly passed as validation anchor.

Test: web-platform-tests/wpt#53353.

Fixes #10155.

Author: annevk

source

@@ -74900,11 +74900,13 @@ dictionary <dfn dictionary>ValidityStateFlags</dfn> {
    <span data-x="face-validation-message">validation message</span>. Otherwise, set
    <var>element</var>'s <span>custom validity error message</span> to the empty string.</p></li>
 
+   <li><p>If <var>anchor</var> is not given, then set it to <var>element</var>.</p></li>
+
+   <li><p>Otherwise, if <var>anchor</var> is not a <span>shadow-including inclusive
+   descendant</span> of <var>element</var>, then throw a <span>"<code>NotFoundError</code>"</span>
+   <code>DOMException</code>.</p></li>
+
    <li><p>Set <var>element</var>'s <span data-x="face-validation-anchor">validation anchor</span> to
-   null if <var>anchor</var> is not given. Otherwise, if <var>anchor</var> is not a
-   <span>shadow-including descendant</span> of <var>element</var>, then throw a
-   <span>"<code>NotFoundError</code>"</span> <code>DOMException</code>. Otherwise, set
-   <var>element</var>'s <span data-x="face-validation-anchor">validation anchor</span> to
    <var>anchor</var>.</p></li>
   </ol>