[css-values] Per WG resolution, define that empty urls must resolve to an invalid resource (not re-request the stylesheet or host page).

Author: tabatkins

css-values/Overview.bs

@@ -539,6 +539,19 @@ Resource Locators: the <<url>> type</h3>
 	(If written as a normal function containing a string, ordinary string escaping rules apply;
 		only newlines and the character used to quote the string need to be escaped.)
 
+	If the value of the ''url()'' is the empty string
+	(like ''url("")'' or ''url()''),
+	the url must resolve to an invalid resource
+	(similar to what the url ''about:invalid'' does).
+
+	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 editting mistakes leaving the ''url()'' value empty,
+	which are almost certain to be invalid resources for whatever the ''url()'' shows up in.
+	Linking on the web platform <em>does</em> allow empty urls,
+	so if/when CSS gains some functionality to control hyperlinks,
+	this restriction can be relaxed in those contexts.
+
 <h4 id="relative-urls">
 Relative URLs</h4>