[css-color-4] percentage opacity is allowed. Fix w3c#3758

Author: svgeesus

css-color-4/Overview.bs

@@ -2459,7 +2459,7 @@ Transparency: the 'opacity' property</h2>
 	Initial: 1
 	Applies to: all elements
 	Inherited: no
-	Percentages: N/A
+	Percentages: map to the range [0,1]
 	Computed value: specified number, clamped to the range [0,1]
 	Animation type: by computed value type
 	</pre>