[selectors] Introduce the :focus-within pseudo class

Author: frivoal

selectors/Overview.bs

@@ -2107,7 +2107,7 @@ The reference element pseudo-class '':scope''</h3>
 User Action Pseudo-classes</h2>
 
 	Interactive user agents sometimes change the rendering in response to user actions.
-	Selectors provides three pseudo-classes for the selection of an element the user is acting on.
+	Selectors provides several pseudo-classes for the selection of an element the user is acting on.
 	(In non-interactive user agents, these pseudo-classes are valid, but never match any element.)
 
 	These pseudo-classes are not mutually exclusive. An element may
@@ -2206,6 +2206,15 @@ The input focus pseudo-class '':focus''</h3>
 	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="the-focus-within-pseudo">
+The Generalized Input Focus Pseudo-Class: '':focus-within''</h3>
+
+	The <dfn id='focus-within-pseudo'>:focus-within</dfn> pseudo-class applies to elements for which the '':focus'' pseudo class applies.
+	Additionaly, the parent of an element that matches '':focus-within'' also matches '':focus-within''.
+
+	Note: When an element that is '':focus-within'' is in a <a>shadow tree</a>,
+	the <a>host element</a> (and its parents) are also in the '':focus-within'' state. (See [[css-scoping-1]])
+
 <h3 id="drag-pseudos">
 The drag-and-drop pseudo-class '':drop'' and '':drop()''</h3>