[css-transitions][css-values][web-animations] Where should animated property type definitions live

[birtles]

We currently define a number of animatable property types in CSS Transitions. These definitions specify what it means to interpolate values of each type. New specs that define new types (e.g. transform lists, filter lists, shapes etc.) are expected to provide their own definitions of interpolation.

Web Animations extends the set of required definitions to include not only interpolation, but also addition, accumulation and distance. It also provides some default definitions for these and a very incomplete set of basic animation types.

In order to implement features like additive animation or paced animation we need to fill out the definitions of addition / distance for many of these types. In general they're not too hard and SVG provides a starting point since addition and pacing are animation features originally used in SVG. Nevertheless, the definitions need to be written up somewhere.

The question is, where do these definitions belong? For now let's assume that specs that introduce new types will provide their own definitions along with the new type, and let's limit the discussion to existing types like length etc.

Some options are:

• Add them to CSS Transitions — feels odd to add these features in when CSS Transitions doesn't use them (or at least not yet, and won't likely use them in the near future, although CSS Animations might)
• Add them to Web Animations — probably involves copying all the interpolation definitions over from CSS Transitions. This somehow seems a little odd for me. It seems like Web Animations should probably just be the model and these definitions should belong in a purely CSS spec.
• Add them to CSS Values (Level 4?)
• Create a new "Animated property types" spec

I spoke to @dbaron yesterday who suggested the last two possibilities. @tabatkins @fantasai @shans I wonder if you have any suggestions / preference?

[shans]

I have a weak preference for adding them to CSS Values level 4, but I'm OK with creating a new specification too.

[tabatkins]

I like having them in V&U. It cuts the Gordian knot of remembering whether they're in Transitions or Animations.

[birtles]

This has now been moved to CSS Values and Units Level 4.