Clarify Window's focus() and blur()

In particular when there's a null browsing context, for #4363.

Tests: web-platform-tests/wpt#15460.

source

@@ -73675,24 +73675,31 @@ END:VCARD</pre>
   <code>Document</code> object, when invoked, must return the result of running the <span>has focus
   steps</span> with the <code>Document</code> object as the argument.</p>
 
-  <p>The <dfn data-dfn-for="Window"><code data-x="dom-window-focus">focus()</code></dfn> method on
-  the <code>Window</code> object, when invoked, must run the <span>focusing steps</span> with the
-  <code>Window</code> object's <span>browsing context</span>. Additionally, if this <span>browsing
-  context</span> is a <span>top-level browsing context</span>, user agents are encouraged to trigger
-  some sort of notification to indicate to the user that the page is attempting to gain focus.</p>
-
-  <p>The <dfn data-dfn-for="Window"><code data-x="dom-window-blur">blur()</code></dfn> method on the
-  <code>Window</code> object, when invoked, provides a hint to the user agent that the script
-  believes the user probably is not currently interested in the contents of the <span>browsing
-  context</span> of the <code>Window</code> object on which the method was invoked, but that the
-  contents might become interesting again in the future.</p>
+  <p>The <dfn data-dfn-for="Window"><code data-x="dom-window-focus">focus()</code></dfn> method,
+  when invoked, must run these steps:</p>
+
+  <ol>
+   <li><p>If this <code>Window</code> object's <span data-x="window bc">browsing context</span> is
+   null, then return.</p></li>
+
+   <li><p>Run the <span>focusing steps</span> with this <code>Window</code> object's <span
+   data-x="window bc">browsing context</span>. Additionally, if that is a <span>top-level browsing
+   context</span>, user agents are encouraged to trigger some sort of notification to indicate to
+   the user that the page is attempting to gain focus.</p></li>
+  </ol>
+
+  <p>The <dfn data-dfn-for="Window"><code data-x="dom-window-blur">blur()</code></dfn> method
+  provides a hint to the user agent that the script believes the user probably is not currently
+  interested in the contents of this <code>Window</code> object's <span data-x="window bc">browsing
+  context</span>, if non-null, but that the contents might become interesting again in the
+  future.</p>
 
   <p>User agents are encouraged to ignore calls to this <code data-x="dom-window-blur">blur()</code>
   method entirely.</p>
 
   <p class="note">Historically, the <code data-x="dom-window-blur">focus()</code> and <code
   data-x="dom-window-blur">blur()</code> methods actually affected the system-level focus of the
-  system widget (e.g. tab or window) that contained the <span>browsing context</span>, but hostile
+  system widget (e.g., tab or window) that contained the <span>browsing context</span>, but hostile
   sites widely abuse this behavior to the user's detriment.</p>
 
   <p>The <dfn><code data-x="dom-focus">focus(<var>options</var>)</code></dfn> method on elements, when