Add a note about late CSP

Closes #7686.

source

@@ -15468,6 +15468,14 @@ 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 an <code data-x="attr-meta-http-equiv">http-equiv</code> attribute in the <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 blocked 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