Add note to optimize contains a header-delivered CSP

index.bs

@@ -543,6 +543,11 @@ spec: INFRA; urlPrefix: https://infra.spec.whatwg.org/
     4.  Return |policies|.
   </ol>
 
+  Note: When <a abstract-op lt="parse a response's Content Security Policies">parsing a response's
+  Content Security Policies</a>, if the resulting |policies| end up containing at least one item,
+  user agents can hold a flag on |policies| and use it to optimize away the [=/contains a
+  header-delivered Content Security Policy=] algorithm.
+
   <h3 id="framework-directives">Directives</h3>
 
   Each <a for="/">policy</a> contains an <a>ordered set</a> of <dfn export>directives</dfn> (its