Add "Should request be allowed to use feature" algorithm

index.bs

@@ -1058,6 +1058,22 @@ partial interface HTMLIFrameElement {
     Note: This algorithm should be called when a feature policy has
     been <a>violated</a>.
   </section>
+  <section>
+    <h3 id="should-request-be-allowed-to-use-feature">Should <var>request</var> be allowed to use <var>feature</var>?</h3>
+    <p>Given a feature (<var>feature</var>) and a  <a for="/">request</a> (<var>request</var>), this algorithm returns <code>true</code> if the request should be allowed to use <var>feature</var>, and <code>false</code> otherwise.</p>
+    <ol>
+      <li>Set |window| to |request|’s <a for="request">window</a>.</li>
+      <li>If |window| is not a {{Window}}, return <code>false</code>.
+       <div class="issue">Feature Policy within `no-window` contexts ({{WorkerGlobalScope}} or {{WorkletGlobalScope}}) is being figured out in <a href="https://github.com/WICG/feature-policy/issues/207">issue #207</a>. After that’s resolved, update this algorithm to allow fetches initiated within these contexts to use policy-controlled features. *Until* that’s resolved, disallow all policy-controlled features (e.g., sending Client Hints to third parties) in these contexts.</div>
+      </li>
+      <li>Set |document| to |window|’s <a>associated `Document`</a>.</li>
+      <li>Let |origin| be |request|’s <a for="request">origin</a>.</li>
+      <li>Let |result| be the result of executing <a href="is-feature-enabled">Is feature enabled in document for origin?</a> on |feature|, |document|, and |origin|.
+      </li>
+      <li>If |result| is "<code>Enabled</code>", return <code>true</code>.</li>
+      <li>Otherwise, return <code>false</code>.</li>
+    </ol>
+  </section>
 </section>
 
 <section>