[selectors] Add >> form of descendant combinator per WG resolution <http://lists.w3.org/Archives/Public/www-style/2014Jun/0106.html>

--HG--
extra : rebase_source : f4471dd2ce2e6741dabe39ba54d3e72a63cf9f91

Author: fantasai

selectors/Overview.bs

@@ -413,10 +413,10 @@ Selectors Overview</h2>
 				<td>3
 		<tbody>
 			<tr>
-				<td><code>E F</code>
+				<td><code>E F</code> <em>or</em> <code>E >> F</code>
 				<td>an F element descendant of an E element
 				<td>[[#descendant-combinators]]
-				<td>1
+				<td>1 <em>or</em> 4
 			<tr>
 				<td><code>E > F</code>
 				<td>an F element child of an E element
@@ -886,7 +886,7 @@ Absolutizing a Relative Selector</h4>
 			only matches elements without a parent.
 
 		<li>
-			Otherwise, if the selector starts with any combinator other than the <a>descendant combinator</a>,
+			Otherwise, if the selector starts with any combinator other than the white space form of the <a>descendant combinator</a>,
 			change the selector to '':not(*)''.
 			<span class='note'>This is the shortest selector that is valid, but guaranteed to match nothing.</span>
 
@@ -898,13 +898,13 @@ Absolutizing a Relative Selector</h4>
 
 	<ol>
 		<li>
-			If the selector starts with a <a>combinator</a> other than the <a>descendant combinator</a>,
+			If the selector starts with a <a>combinator</a> other than the white space form of the <a>descendant combinator</a>,
 			prepend '':scope'' as the initial <a>compound selector</a>.
 
 		<li>
 			Otherwise, if the selector does not contain any instance of the '':scope'' pseudo-class
 			(either at the top-level or as an argument to a functional pseudo-class),
-			prepend '':scope'' followed by the <a>descendant combinator</a>.
+			prepend '':scope'' followed by the white space form of the <a>descendant combinator</a>.
 
 		<li>
 			Otherwise, the selector is already absolute.
@@ -2893,15 +2893,19 @@ Typed Child-indexed Pseudo-classes</h3>
 Combinators</h2>
 
 <h3 id="descendant-combinators">
-Descendant combinator (<code> </code>)</h3>
+Descendant combinator (<code> </code>) <em>or</em> (<code>>></code>)</h3>
 
 	At times, authors may want selectors to describe an element that is
 	the descendant of another element in the document tree (e.g., "an
 	<a element>em</a> element that is contained within an <a element>H1</a> element").
-	Descendant combinators express such a relationship. A
-	<dfn export>descendant combinator</dfn> is <a href="#whitespace">whitespace</a> that
-	separates two <a>compound selectors</a>.  A selector of the form
-	''A B'' represents an element <code>B</code> that is an
+	The <dfn export>descendant combinator</dfn> expresses such a relationship.
+	It has two syntactic forms:
+	<ol>
+	  <li><a href="#whitespace">whitespace</a> that separates two <a>compound selectors</a> <em>or</em>
+	  <li>A doubled child selector (<code>>></code>)
+	</ol>
+	
+	A selector of the form ''A B'' or ''A >> B''' represents an element <code>B</code> that is an
 	arbitrary descendant of some ancestor element <code>A</code>.
 
 	<div class="example">
@@ -2937,6 +2941,12 @@ Descendant combinator (<code> </code>)</h3>
 		<pre>div p *[href]</pre>
 	</div>
 
+	Note: Note that the whitespace form has existed since CSS Level 1,
+	whereas the <code>>></code> form is introduced in Level 4.
+	It was introduced to give the descendant combinator a visible, non-whitespace form
+	and to bridge the gap between the <a>child combinator</a> (<code>></code>)
+	and the shadow-piercing descendant combinator (<code>>>></code>) [[CSS-SCOPING-1]].
+
 <h3 id="child-combinators">
 Child combinator (<code>></code>)</h3>