Address review comments

html/webappapis/dynamic-markup-insertion/opening-the-input-stream/beforeunload.window.js

@@ -6,7 +6,9 @@ async_test(t => {
     frame.contentWindow.onbeforeunload = t.unreached_func("beforeunload should not be fired");
     frame.contentDocument.open();
     t.step_timeout(t.step_func_done(() => {
-      frame.contentDocument.close();
+      // If the beforeunload event has still not fired by this point, we
+      // consider the test a success. `frame.remove()` above will allow the
+      // `load` event to be fired on the top-level Window.
     }), 500);
   });
-}, "document.open() and the beforeunload event");
+}, "document.open() should not fire a beforeunload event");