[css-values-4] Add generic interpolation function. #581 #2854

Author: fantasai

css-values-4/Overview.bs

@@ -449,6 +449,44 @@ Combining Values: Interpolation, Addition, and Accumulation
 	If a value type does not define a specific procedure for accumulation,
 	its accumulation operation is identical to addition.
 
+
+Representing Interpolated Values: the ''mix()'' notation
+
+	[=Interpolation=] of two values can be represented
+	by the mix() [=functional notation=],
+	whose syntax is defined as follows:
+
+	
+		mix( <> ; <> ; <
+	
+
+	
+		<>
+		
+			Represents the interpolation point
+			as progress from <> to <>.
+
+		<>
+		
+			The value at the “start” (0%) of the interpolation range.
+
+		<>
+		
+			The value at the “end” (100%) of the interpolation range.
+	
+
+	Note: This [=functional notation=] uses semicolons to separate arguments
+	rather than the more typical comma
+	because the values themselves can contain commas.
+
+	A ''mix()'' notation is invalid if either its <> or <>
+	is invalid if substituted in its place,
+	or if the property using it is [=not animatable=].
+
+	ISSUE: Define more precisely where mix() is allowed.
+	Is it a top-level functional notation only?
+	Or can it be nested more deeply in the grammar somehow?
+
 
 Range Checking