[css-color] Since all implementations agree that rgb() is always sRGB gamut, go ahead and define that all arguments are just clamped to [0%, 100%] range. Fixes w3c#288.

Author: tabatkins

css-color/Overview.bs

@@ -190,23 +190,17 @@ The RGB functions: ''rgb()'' and ''rgba()''</h3>
 	not just <<integer>>s,
 	for authoring convenience.
 
-	Some devices can output colors technically outside of the sRGB gamut,
-	represented by channels with values less than ''0%'' or greater than ''100%''.
-	For this reason, values outside of the 0%-100% range are allowed,
-	but are clamped to the device's gamut.)
-
-	Issue: The transfer function is undefined outside the range 0% to 100%.
-
 	The final argument, the <<alpha-value>>, specifies the alpha of the color.
 	If given as a <<number>>, the useful range of the value is ''0''
 	(representing a fully transparent color)
 	to ''1''
 	(representing a fully opaque color).
 	If given as a <<percentage>>, ''0%'' represents a fully transparent color,
 	while ''100%'' represents a fully opaque color.
+	If omitted, it defaults to ''100%''.
+
 	Values outside these ranges are not invalid,
 	but are clamped to the ranges defined here at computed-value time.
-	If omitted, it defaults to ''100%''.
 
 	Note: For legacy reasons, the ''rgb()'' function and ''rgba()'' functions both exist,
 	but are aliases of each other.