[css-images-3] Must->should to match the WG resolution. w3c#383

Author: fantasai

css-images-3/Overview.bs

@@ -177,13 +177,13 @@ Image References: the ''url()'' notation {#url-notation}
 	a <<url>> value that can be interpreted
 	as either an <<image>> or an element reference.
 	If an [=ambigous image URL=] is a [[css-values-3#local-urls|fragment-only URL]],
-	then it must be treated as an element reference.
+	then it should be treated as an element reference.
 	Otherwise, if the [=ambiguous image URL=] has a fragment
 	that references an element in the resource
 	that is an appropriate type of element for the context in which the <<url>> appears
 	(such as a <{mask}> element for the 'mask-image' property),
-	it must be interpreted as an element reference.
-	Otherwise, it must be treated as an <<image>>.
+	it should be interpreted as an element reference.
+	Otherwise, it should be treated as an <<image>>.
 
 	Specs using the [=ambiguous image URL=] concept
 	must define what elements are valid references for the URL,