Add an introduction for scoped custom element registries

Author: annevk

source

@@ -73683,6 +73683,20 @@ console.log(plasticButton.outerHTML); // will output '<button is="plastic-but
   console.assert(outOfDocument instanceof ExampleElement);
 &lt;/script></code></pre>
 
+  <h5>Scoped custom element registries</h5>
+
+  <p>To allow multiple libraries to co-exist without explicit coordination,
+  <code>CustomElementRegistry</code> can be used in a scoped fashion as well.</p>
+
+  <pre><code class="javascript">const scoped = new CustomElementRegistry();
+scoped.define("example-element", ExampleElement);
+
+const element = document.createElement("example-element", { customElementRegistry: scoped });</code></pre>
+
+  <p>A node with an associated scoped <code>CustomElementRegistry</code> will use that registry for
+  all its operations, such as when invoking <code
+  data-x="dom-Element-setHTMLUnsafe">setHTMLUnsafe()</code>.</p>
+
   <h5>Exposing custom element states</h5>
 
   <p>Built-in elements provided by user agents have certain states that can change over time