[css-grid-1] Specify that grid-template <string> whitespace is normalized to just a single space between cell tokens. Fixes #4335.

Author: tabatkins

css-grid-1/Overview.bs

@@ -2085,10 +2085,13 @@ Named Areas: the 'grid-template-areas' property</h3>
 		</pre>
 	</div>
 
-	Both the <a>specified value</a> and <a>computed value</a>
-	of a <<string>> value of 'grid-template-areas' serializes
-	each <a>null cell token</a> as a single "." (U+002E FULL STOP)
-	and each sequence of <a>whitespace</a> as a single space (U+0020 SPACE).
+<h4 id="serialize-template">
+Serialization Of Template Strings</h4>
+
+	When serializing either the [=specified value|specified=] or [=computed value=] of a <<string>> value of 'grid-template-areas',
+	each [=null cell token=] is serialized as a single "." (U+002E FULL STOP),
+	and consecutive cell tokens are separated by a single space (U+0020 SPACE),
+	with all other white space elided.
 
 <h4 id="implicit-named-lines">
 Implicit Named Lines</h4>