[css-images-3] Note that we might change gradient interpolation to be smarter later.

Author: fantasai

css-images-3/Overview.bs

@@ -1710,11 +1710,15 @@ Interpolating <<gradient>> {#interpolating-gradients}
 	If the two gradients satisfy all of those constraints,
 	they must be interpolated as described below.
 	If they fail the third one only,
-	they must be abruptly transitioned at 50%.
+	they must be abruptly transitioned at 50%
+	(unless otherwise specified by a future specification).
 	If they fail either of the first two constraints,
 	they must be interpolated using ''cross-fade()''
 	as for generic images.
 
+	Note: The abrupt transition at 50% is so that content will not rely on cross-fading,
+	and smarter interpolation rules can be added for this case in the future.
+
 	1. Convert both the start and end gradients to their explicit forms:
 
 		: For linear gradients: