Add 'UA style sheet' anchor.

Author: tabatkins

css-cascade/Overview.bs

@@ -763,7 +763,7 @@ Cascading Origins</h3>
 			or the user agent may provide an interface that generates a user style sheet
 			(or behaves as if it did).
 
-		<dt><dfn export id='cascade-origin-ua' lt="user agent origin|ua origin">User Agent Origin</dfn>
+		<dt><dfn export id='cascade-origin-ua' lt="user agent origin|ua origin|user agent style sheet|ua style sheet">User Agent Origin</dfn>
 		<dd>
 			Conforming user agents must apply a default style sheet
 			(or behave as if they did).