[selectors] make the :focus pseudo class propagate the same was as the :active pseudo class does, except for matching on the parent element.

Author: frivoal

selectors/Overview.bs

@@ -2202,6 +2202,10 @@ The input focus pseudo-class '':focus''</h3>
 	which elements can acquire '':focus''.
 	For example, [[HTML5]] defines a <a href="http://www.whatwg.org/html/selectors.html#selector-active">list of activatable elements</a>.
 
+	If the host language has defined additional ways by which an element can match '':active'',
+	the same ways must apply to elements matching '':focus'' as well,
+	except that the parent of an element that matches '':focus'' must not match '':focus''.
+
 <h3 id="drag-pseudos">
 The drag-and-drop pseudo-class '':drop'' and '':drop()''</h3>