[css-syntax] Clarify that whitespace may be defined to be significant in rule grammars.

Author: tabatkins

css-syntax/Overview.html

@@ -4033,7 +4033,9 @@ <h2 id=rule-defs><span class=secno>7. </span> Defining Grammars for Rules
    <code>'+'</code>. Similarly, the 〈[〉 and 〈]〉s must be written in
    single quotes, as they're used by the syntax of the grammar itself to
    group clauses. 〈whitespace〉 is never indicated in the grammar;
-   〈whitespace〉s are allowed before, after, and between any two tokens.
+   〈whitespace〉s are allowed before, after, and between any two tokens,
+   unless explicitly specified otherwise in prose definitions. (For example,
+   if the prelude of a rule is a selector, whitespace is significant.)
 
   <p> When defining a function or a block, the ending token must be specified
    in the grammar, but if it's not present in the eventual token stream, it

css-syntax/Overview.src.html

@@ -2990,7 +2990,10 @@ <h2 id='rule-defs'>
 		Similarly, the 〈[〉 and 〈]〉s must be written in single quotes,
 		as they're used by the syntax of the grammar itself to group clauses.
 		〈whitespace〉 is never indicated in the grammar;
-		〈whitespace〉s are allowed before, after, and between any two tokens.
+		〈whitespace〉s are allowed before, after, and between any two tokens,
+		unless explicitly specified otherwise in prose definitions.
+		(For example, if the prelude of a rule is a selector,
+		whitespace is significant.)
 
 	<p>
 		When defining a function or a block,