[css-images-4] Specify that image-set() is an invalid image if you remove all the options. w3c#8266

Author: tabatkins

css-images-4/Overview.bs

@@ -247,6 +247,8 @@ Resolution/Type Negotiation: the ''image-set()'' notation {#image-set-notation}
 		that specify an unknown or unsupported MIME type in their ''type()'' value.
 	2. Second, remove any <<image-set-option>>s from the list
 		that have the same <<resolution>> as a previous option in the list.
+	3. If there are no <<image-set-option>> left at this point,
+		the function represents an [=invalid image=].
 	3. Finally, among the remaining <<image-set-option>>s,
 		make a UA-specific choice of which to load,
 		based on whatever criteria deemed relevant