Scope non-scoped (global) registries to a single document

Let the global custom element registry not escape its document for now. We throw an exception to give us some wiggle room in the future.

DOM issue and PR: whatwg/dom#1390 & whatwg/dom#1392.

Tests: TODO.

Author: annevk

source

@@ -73638,7 +73638,12 @@ dictionary <dfn dictionary>ElementDefinitionOptions</dfn> {
 
    <dt><code data-x=""><var>registry</var>.<span subdfn data-x="dom-CustomElementRegistry-initialize">initialize</span>(<var>root</var>)</code></dt>
    <dd>Each <span>inclusive descendant</span> of <var>root</var> with a null registry will have it
-   updated to this <code>CustomElementRegistry</code> object.</dd>
+   updated to this <code>CustomElementRegistry</code> object. A
+   <span>"<code>NotSupportedError</code>"</span> <code>DOMException</code> will be thrown if this
+   <code>CustomElementRegistry</code> object is not for scoped usage and <var>root</var> is either a
+   <code>Document</code> node or <var>root</var>'s <span>node document</span>'s <span
+   data-x="document-custom-element-registry">custom element registry</span> is not this
+   <code>CustomElementRegistry</code> object.</dd>
   </dl>
 
   <p w-nodev>The <dfn
@@ -73987,6 +73992,12 @@ console.assert(el instanceof SpiderMan);    // upgraded!</code></pre>
   steps are:</p>
 
   <ol>
+   <li><p>If <span>this</span>'s <span>is scoped</span> is false and <var>root</var> is either a
+   <code>Document</code> node or <var>root</var>'s <span>node document</span>'s <span
+   data-x="document-custom-element-registry">custom element registry</span> is not
+   <span>this</span>, then throw a <span>"<code>NotSupportedError</code>"</span>
+   <code>DOMException</code>.</p></li>
+
    <li><p>If <var>root</var> is a <code>Document</code> node whose <span
    data-x="document-custom-element-registry">custom element registry</span> is null, then set
    <var>root</var>'s <span data-x="document-custom-element-registry">custom element registry</span>