[css-images-4] Add the -webkit-image-set() alias, per #6285.

Author: tabatkins

css-images-4/Overview.bs

@@ -2210,6 +2210,14 @@ Serialization {#serialization}
 		linear-gradient(red, yellow, black 100px)
 	
 
+
+Appendix A: Deprecated Features and Aliases
+
+Implementations must accept -webkit-image-set()
+as a parse-time alias of ''image-set()''.
+(It's a valid value, with identical arguments to ''image-set()'',
+and is turned into ''image-set()'' during parsing.)
+
 Privacy and Security Considerations {#priv-sec}
 ==============================================
 
@@ -2226,6 +2234,7 @@ Changes Since the 
 - Added start angles to conic gradients
 - The position(s) of a color stop can now come before the color
 - Text that is identical to [[css3-images]] has been replaced with a reference to [[css3-images]].
+- Added the ''-webkit-image-set()'' alias.
 
 
 Changes Since Level 3