[css-images-4] Add placeholder issues for defining simplification of cross-fade() arguments/nesting, per WG resolution.

Author: tabatkins

css-images-4/Overview.bs

@@ -531,6 +531,19 @@ Combining images: the ''cross-fade()'' notation {#cross-fade-function}
 		8. Return |final image|.
 	</div>
 
+### Simplifying Complex ''cross-fade()'' ### {#cross-fade-complex}
+
+	Issue: Per WG resolution,
+	define a notion of "equality" for images,
+	and combine "same" images at computed-value time,
+	summing their percentages.
+
+	Issue: Per WG resolution,
+	simplify directly-nested ''cross-fade()'' at computed-value time
+	by just distributing the percentage and flattening;
+	''cross-fade(A 10%, cross-fade(B 30%, C 70%) 90%)''
+	becomes ''cross-fade(A 10%, B 27%, C 63%)''.
+
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