[css-counter-styles] @counter-style with missing descriptors just fails to define a counter style, but is otherwise treated as valid. Fixes #1682.

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -1039,7 +1039,8 @@ Marker characters: the 'symbols' and 'additive-symbols' descriptors</h3>
 	''cyclic'', ''system/numeric'', ''system/alphabetic'', ''symbolic'', or ''system/fixed'',
 	or a valid 'additive-symbols' descriptor
 	if the counter system is ''additive'';
-	otherwise, the ''@counter-style'' is invalid and must be ignored.
+	otherwise, the ''@counter-style'' does not define a <a>counter style</a>
+	(but is still a valid <a>at-rule</a>).
 
 	Some counter systems specify that the 'symbols' descriptor must have at least two entries.
 	If the counter style's system is such,