Return null when window.open() is popup blocked

Follow up to whatwg#1848, fixes whatwg#1851.

source

@@ -78688,7 +78688,7 @@ callback <dfn>FrameRequestCallback</dfn> = void (<span>DOMHighResTimeStamp</span
     <p>Otherwise, apply <span>the rules for choosing a browsing context given a browsing context
     name</span> using <var>target</var> as the name and <var>source browsing context</var> as the
     context in which the algorithm is executed. If this results in there not being a chosen browsing
-    context, then abort these steps. Otherwise, let <var>target browsing context</var> be the
+    context, then return null. Otherwise, let <var>target browsing context</var> be the
     <span>browsing context</span> so obtained.</p>
 
    </li>
@@ -78757,9 +78757,9 @@ callback <dfn>FrameRequestCallback</dfn> = void (<span>DOMHighResTimeStamp</span
     <ol>
 
      <li><p>If the result of <span data-x="split a string on commas">splitting <var>features</var>
-     on commas</span> contains the token "<code data-x="">noopener</code>", <span data-x="disowned
-     its opener">disown <var>target browsing context</var>'s opener</span>, and return <code
-     data-x="">null</code>.</p></li>
+     on commas</span> contains the token "<code data-x="">noopener</code>", then <span
+     data-x="disowned its opener">disown <var>target browsing context</var>'s opener</span> and
+     return null.</p></li>
 
      <li><p>Otherwise, return the <code>WindowProxy</code> object of <var>target browsing
      context</var>.</p></li>