Add note about late CSP

Closes whatwg#7686

source

@@ -15404,6 +15404,14 @@ people expect to have work and what is necessary.
     data-x="attr-meta-content">content</code> attribute will be <span
     data-x="enforce the policy">enforced</span> upon the current document. <ref spec=CSP></p>
 
+    <p class="note">At the time of inserting the <code>meta</code> element to the document, it is
+    possible that some resources have already been fetched. For example, images might be stored in
+    the <span>list of available images</span> prior to dynamically inserting a <code>meta</code>
+    element with a <span data-x="attr-meta-http-equiv-content-security-policy">Content security
+    policy state</span>. Resources that have already been fetched are not guaranteed to be
+    protected by a <span>Content Security Policy</span> that's
+    <span data-x="enforce the policy">enforced</span> late.</p>
+
     <div class="example">
 
      <p>A page might choose to mitigate the risk of cross-site scripting attacks by preventing the