[css-values-4] Require <<ratio>> to be serialized with both components, per WG resolution. w3c#5561

Author: fantasai

css-values-4/Overview.bs

@@ -1250,8 +1250,9 @@ Ratios: the <<ratio>> type</h3>
 		<<ratio>> = <<number [0,Infinity]>> [ / <<number [0,Infinity]>> ]?
 	</pre>
 
-	If only one <<number>> is provided,
-	the second defaults to ''1''.
+	The second <<number>> is optional,
+	defaulting to ''1''.
+	However, <<ratio>> is always serialized with both components.
 
 	The computed value of a <<ratio>> is the pair of numbers provided,
 	unless both numbers are zero,