991950 - custom properties ("css variables") should be animatable/transitionable with the "flip value at 50%" interpolation behaviour

Status: NEW

(Core :: CSS Parsing and Computation, defect)

Description

[Cameron McCormack (:heycam)]

The CSS Variables spec now says:

Custom properties are ordinary properties, so they can be declared on any element, are resolved with the normal inheritance and cascade rules, can be made conditional with @media and other conditional rules, can be used in HTML’s style attribute, can be read or set using the CSSOM, etc. Notably, they can even be transitioned or animated, but since the UA has no way to interpret their contents, they always use the "flips at 50%" behavior that is used for any other pair of values that can’t be intelligently interpolated.

So we should support animating them like this.

Comment 1

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

This is really more general, though, since we need to support this for all non-animatable properties.

Comment 3

[Greg Whitworth]

@dbaron Out of curiosity, do you plan to implement this for both known properties as well as unknown properties? For example, within transitions do you plan to make it so that it does a 50% flip rather than that of interpolating even though you know how to after substitution? Here's an example where Chrome/FF will interpolate a known property post variable substitution. http://jsbin.com/wumufepexo/edit?html,output