Use DOM's "replace all" primitive in document.open()

This makes it clearer what is expected with respect to mutation observers.

source

@@ -90648,9 +90648,8 @@ document.body.appendChild(frame)</pre>
    <!-- removes callbacks that fired between this algorithm starting and the times and databases
         being aborted in the "unload" step above -->
 
-   <li><p>Remove all child nodes of <var>document</var>, without firing any mutation
-   events.</p></li>
-   <!-- as of 2009-03-30, only WebKit fired mutation events here. -->
+   <li><p><span data-x="concept-node-replace-all">Replace all</span> with null within
+   <var>document</var>, without firing any mutation events.</p></li>
 
    <li>
     <p>Call the JavaScript <span