check and consume user activation

index.html

@@ -552,6 +552,11 @@ <h2>
         The <dfn>subscribe</dfn> method when invoked MUST run the following steps:
       </p>
       <ol>
+        <li>If the [=relevant global object=] of [=this=] does not have [=transient activation=],
+        return [=a promise rejected with=] with a {{"NotAllowedError"}} {{DOMException}}.
+        </li>
+        <li>[=Consume user activation=] of [=this=]'s [=relevant global object=].
+        </li>
         <li>Let |promise| be <a>a new promise</a>.
         </li>
         <li>Return |promise| and continue the following steps asynchronously.