interpolate() is defined to possibly take arguments that are not animatable.

The interpolate() notation is a <whole-value>. Additionally, if any of its <whole-value> arguments are not animatable, the notation is invalid.

The definition of not animatable refers to properties, not values:

The property is not animatable. It is not processed when listed in an animation keyframe, and is not affected by transitions.

Are you considering defining property animation types based on their specified value and updating Web Animations accordingly? Do you have an example of a property that is not animatable with some type of value?

(edit) replaced mix() with interpolate(), following the corresponding change introduced by 5fbd75b.