[css-values] How should media-progress() and container-progress() serialize?

[weinig]

The current spec does not specify how media-progress() and container-progress() serialize. Since progress() is a "math function", its serialization can follow https://drafts.csswg.org/css-values-4/#serialize-a-math-function, but the other two are not.

If they can be fully simplified at parse time, should they serialize as a in specified value serializations?