Status update:
Have a WIP patch that adds support for animating <color> custom properties: https://codereview.chromium.org/2564793002
Major things still to do:
- Support all <syntax>es.
- Defer tokenisation of animated custom property values until they're actually needed.
- Integrate animated custom property value application with variable resolution ordering. At the moment we apply animated custom property values in a random (hash) order We need to apply them in the order defined by the way they reference each other. The logic for this is already done in CSSVariableResolver, we just need to reuse it somehow.
I believe that this is done nowadays (at the very least the reproduction in the first comment works - https://output.jsbin.com/gawatat). But cc andruud@ to check that there isn't work remaining on animating registered custom properties that should be tracked here.
This is blocked on animation of transition custom properties.
https://github.com/w3c/csswg-drafts/issues/2854
Basically it's currently impossible to serialise a computed value for an animating registered custom transform property that uses matrix interpolation and percentage translations without invoking layout prior to completing style resolve.
Comment 1 by alancutter@chromium.org
, Dec 7 2016