diff --git a/source b/source
index d99b6a4652e..cc2a9b7a388 100644
--- a/source
+++ b/source
@@ -59047,6 +59047,7 @@ interface <dfn>HTMLTemplateElement</dfn> : <span>HTMLElement</span> {
   [<span>HTMLConstructor</span>] constructor();
 
   readonly attribute <span>DocumentFragment</span> <span data-x="dom-template-content">content</span>;
+  readonly attribute ShadowRoot? <span data-x="dom-template-shadowroot">shadowRoot</span>;
 };</code></pre>
 </dd>
    <dd w-dev>Uses <code>HTMLTemplateElement</code>.</dd>
@@ -59065,6 +59066,15 @@ interface <dfn>HTMLTemplateElement</dfn> : <span>HTMLElement</span> {
   of the <code>template</code> element's content model, since its content model is defined as <span
   data-x="concept-content-nothing">nothing</span>.</p>
 
+  <p class="note">The <code data-x="dom-template-shadowroot">shadowRoot</code> attribute on HTMLTemplateElement is present for feature detection
+  of declarative Shadow DOM:</p>
+
+<pre><code class="html" data-x="">&lt;script>
+  function supportsDeclarativeShadowDOM() {
+    return HTMLTemplateElement.prototype.hasOwnProperty("shadowRoot");
+  }
+&lt;/script></code></pre>
+
   <div class="example" id="template-example">
 
    <p>For example, consider the following document:</p>
@@ -59193,6 +59203,9 @@ interface <dfn>HTMLTemplateElement</dfn> : <span>HTMLElement</span> {
   property is true, then the <code data-x="dom-template-content">content</code> attribute must return
   null instead.</p>
 
+  <p>The <dfn><code data-x="dom-template-shadowroot">shadowroot</code></dfn> IDL attribute must return
+  its base Element's <a href="https://dom.spec.whatwg.org/#dom-element-shadowroot">shadowroot</a> value.
+
   <hr>
 
   <p>The <span data-x="concept-node-clone-ext">cloning steps</span> for a <code>template</code>