Clarify that var() must be valid inside of custom properties, or else the property is invalid.

Author: tabatkins

css-variables/Overview.bs

@@ -137,6 +137,10 @@ Custom Property Value Syntax</h3>
 	unmatched <<)-token>>, <<]-token>>, or <<}-token>>,
 	or top-level <<semicolon-token>> tokens or <<delim-token>> tokens with a value of "!".
 
+	In addition, if the value of a <a>custom property</a> contains a ''var()'' reference,
+	the ''var()'' reference must be valid according to the specified ''var()'' grammar.
+	If not, the <a>custom property</a> is invalid and must be ignored.
+
 	Note: This definition,
 	along with the general CSS syntax rules,
 	implies that a custom property value never includes an unmatched quote or bracket,
@@ -609,7 +613,7 @@ Variables in Shorthand Properties</h3>
 	if an API allows them to be observed.
 
 	Similarly,
-	while [[CSSOM-1]] defines that shorthand properties are serialized
+	while [[CSSOM]] defines that shorthand properties are serialized
 	by appropriately concatenating the values of their corresponding longhands,
 	shorthands that are specified with explicit ''var()'' functions
 	must serialize to the original, ''var()''-containing value.
@@ -667,6 +671,7 @@ Changes since the May 6 2014 Last Call Working Draft</h2>
 	* Serialization of longhands when shorthand uses a variable was defined.
 	* Link to DOM's definition of "case-sensitive".
 	* Added example of using variables with '':lang()'' to do simple i18n.
+	* Clarified that usage of ''var()'' in a custom property must be valid per the ''var()'' grammar.
 
 
 <h2 id="acks">