[css-text-4] add preserve-spaces to describe an option from SVG

Author: astearns

css-text-4/Overview.bs

@@ -35,7 +35,7 @@ White Space Collapsing: the 'text-space-collapse' property</h3>
 
 	<pre class="propdef">
 	Name: text-space-collapse
-	Value: collapse | discard | preserve | preserve-breaks
+	Value: collapse | discard | preserve | preserve-breaks | preserve-spaces 
 	Initial: collapse
 	Applies to: all elements
 	Inherited: yes
@@ -69,6 +69,14 @@ White Space Collapsing: the 'text-space-collapse' property</h3>
 			This value collapses white space as for ''collapse'', but preserves
 			line feeds as forced line breaks.
 
+		<dt><dfn>preserve-spaces</dfn>
+		<dd>
+			This value prevents user agents
+			from collapsing sequences of white space, 
+			and converts tabs and line feeds to spaces. 
+			(This value is intended to match the behavior 
+			of xml:space="preserve" in SVG)
+
 		<dt><dfn>discard</dfn>
 		<dd>
 			This value directs user agents to "discard" all white space in the