[selectors-4] Add :target-within, resolves w3c#457

Author: LeaVerou

selectors-4/Overview.bs

@@ -242,6 +242,11 @@ Selectors Overview</h2>
 				<td>an E element being the target of the referring URL
 				<td>[[#the-target-pseudo]]
 				<td>3
+			<tr>
+				<td><code>E:target-within</code>
+				<td>an E element that is the target of the referring URL or contains an element that does.
+				<td>[[#the-target-within-pseudo]]
+				<td>4
 			<tr>
 				<td><code>E:scope</code>
 				<td>an E element being a designated reference element
@@ -1971,6 +1976,15 @@ The Target Pseudo-class: '':target''</h3>
 		</pre>
 	</div>
 
+<h3 id="the-target-within-pseudo">
+The Generalized Target Pseudo-class: '':target-within''</h3>
+
+	The <dfn id='target-within-pseudo'>:target-within</dfn> pseudo-class applies to elements for which the '':target'' pseudo class applies.
+
+	An element also matches '':target-within''
+	if one of its descendants in the <a>flat tree</a>
+	(including non-element nodes, such as text nodes)
+	matches the conditions for matching '':target''.
 
 <h3 id="the-scope-pseudo">
 The Reference Element Pseudo-class: '':scope''</h3>