[css-values] Per WG resolution, empty urls() serialize as url(''). #6447

Author: tabatkins

css-values-3/Overview.bs

@@ -665,6 +665,9 @@ Empty URLs</h4>
 	the url must resolve to an invalid resource
 	(similar to what the url ''about:invalid'' does).
 
+	Its computed value is ''url("")'',
+	and it must serialize as such.
+
 	Note: This matches the behavior of empty urls for embedded resources elsewhere in the web platform,
 	and avoids excess traffic re-requesting the stylesheet or host document
 	due to editing mistakes leaving the ''url()'' value empty,

css-values-4/Overview.bs

@@ -993,6 +993,9 @@ Empty URLs</h4>
 	the url must resolve to an invalid resource
 	(similar to what the url ''about:invalid'' does).
 
+	Its computed value is ''url("")'',
+	and it must serialize as such.
+
 	Note: This matches the behavior of empty urls for embedded resources elsewhere in the web platform,
 	and avoids excess traffic re-requesting the stylesheet or host document
 	due to editing mistakes leaving the ''url()'' value empty,