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

Author: tabatkins

css-images-4/Overview.bs

@@ -2210,6 +2210,14 @@ Serialization {#serialization}
 		<pre>linear-gradient(red, yellow, black 100px)</pre>
 	</div>
 
+<h2 id=deprecated class=no-num>
+Appendix A: Deprecated Features and Aliases</h2>
+
+Implementations must accept <dfn>-webkit-image-set()</dfn>
+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 <a href="https://www.w3.org/TR/2012/WD-css4-images-20120911/">
 - Added start angles to <a href="#conic-gradients">conic gradients</a>
 - 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.
 
 <h3 class="no-num" id="changes-3">
 Changes Since Level 3</h3>