Editorial: add note why error pages still fire a load event

Author: TimvdLippe

source

@@ -112838,8 +112838,15 @@ new PaymentRequest(…); // Allowed to use
    the normal <code>Document</code> rendering rules, or mutate <var>document</var> until it
    represents the content the user agent wants to render.</p></li>
 
-   <li><p>Act as if the user agent had <span data-x="stop parsing">stopped parsing</span>
-   <var>document</var>.</p></li>
+   <li>
+    <p>Act as if the user agent had <span data-x="stop parsing">stopped parsing</span>
+    <var>document</var>.</p>
+
+    <p class="note">This is done to avoid leaking information to the parent page about whether a
+    navigation has been blocked by CSP, or if the response was a <span>network error</span>. In the
+    case where the container is an <code>iframe</code> element, <span data-x="stop parsing">stopping
+    parsing</span> causes the <span>iframe load event steps</span> to run.</p>
+   </li>
 
    <li><p>Return <var>document</var>.</p></li>
   </ol>