[css-variables-1] var()s in longhands also block shorthands from being serialized. w3c#6253

Author: tabatkins

css-variables-1/Overview.bs

@@ -796,6 +796,7 @@ Variables in Shorthand Properties</h3>
 	Otherwise,
 	if any of the component [=longhand properties=] for a given [=shorthand=]
 	are [=pending-substitution values=],
+	or contain ''var()'' functions of their own that have not yet been substituted,
 	the [=shorthand property=] must serialize to the empty string.
 
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