[css-values-4] Specify that hash-only URLs resolve against the node tree, not the document (so they're shadow-scoped), per WebAppsWG discussion. w3c#3320

Author: tabatkins

css-values-4/Overview.bs

@@ -953,7 +953,8 @@ Fragment URLs</h5>
 
 	When matching a ''url()'' with the <a>local url flag</a> set,
 	ignore everything but the URL's fragment,
-	and resolve that fragment against the current document that relative URLs are resolved against.
+	and resolve that fragment against the [=node tree=]
+	of the stylesheet's [=CSSStyleSheet/owner node=].
 	This reference must always be treated as same-document
 	(rather than cross-document).