Regression fix: run iframe load event steps immediately

This regressed in 61da493 when moving away from "asynchronously" and "synchronously".

Fixes #3357.

Author: annevk

source

@@ -29655,7 +29655,7 @@ interface <dfn>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
   location.assign() method -->
 
   <p>When a <code>Document</code> in an <code>iframe</code> is marked as <span>completely
-  loaded</span>, the user agent must run the <span>iframe load event steps</span> <span>in parallel</span>.</p>
+  loaded</span>, the user agent must run the <span>iframe load event steps</span>.</p>
 
   <p class="note">A <code data-x="event-load">load</code> event is also fired at the
   <code>iframe</code> element when it is created if no other data is loaded in it.</p>