Thanks! :D I wanted really badly to be able to do it, so I read the spec very carefully top to bottom a handful of times. And suddenly one day I just realized it. Dove right in and after a bit of progress, I discovered all the logic that was possible too.

I had already been using spaces as a fallback value to do dynamic composition of standard properties ( https://twitter.com/James0x57/status/1233246818118533120 ) so it wasn't too far off to combine it with the "initial" behavior that causes the entire var dependency tree to evaluate to initial, allowing fallback behavior from any point in the tree. :)