[css-transitions] Allow transitions to work at used value time #10204
Description
The big issue of #626 that limits the given solutions to certain use cases seems to be that transitions are specified to happen at computed value time.
So, what if we allow authors to change at what step of the value processing transitions are working. I. e. allow to specify that transitions should be based on used values instead.
Not only would this solve the use case of interpolating between dimensional values and keywords like auto but also transitions between two keywords (even if it's the same like between auto and auto at two different times).
Obviously, this has some big side effects and issues that need to be solved, as mentioned by @tabatkins and others in the other issue.
Though obviously it is possible under certain circumstances, as Tab also noted for Anchor Positioning and Container Queries.
This issue is meant to explore the restrictions required to provide authors a general way to opt into transitions between used values instead of computed values.
Sebastian