[css-variables] Fix how shorthands serialize, per jdaggett's comments.

--HG--
extra : rebase_source : ec7efc64d225a3167ee862372f82298db92724a8

Author: tabatkins

css-variables/Overview.bs

@@ -591,9 +591,12 @@ Variables in Shorthand Properties</h3>
 	Similarly,
 	while [[CSSOM]] defines that shorthand properties are serialized
 	by appropriately concatenating the values of their corresponding longhands,
-	shorthands containing ''var()'' functions must instead retain their original, ''var()''-containing, value.
-	If serializing a shorthand's value would involve serializing a <a>pending-substitution value</a>,
-	the shorthand must instead be serialized by serializing its original value.
+	shorthands that are specified with explicit ''var()'' functions
+	must serialize to the original, ''var()''-containing value.
+	For other shorthands,
+	if any of the longhand subproperties for that shorthand have <a>pending-substitution values</a>
+	then the serialized value of the shorthand must be the empty string.
+
 
 
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