[css-color] Fix computed/animated value for opacity

Author: fantasai

css-color/Overview.bs

@@ -2701,7 +2701,8 @@ Transparency: the 'opacity' property</h2>
 	Inherited: no
 	Percentages: N/A
 	Media: visual
-	Computed value: The specified value, clamped to the range [0,1].
+	Computed value: the specified value converted to a number, clamped to the range [0,1]
+	Animation type: as number
 	</pre>
 
 	<dl>