[css-values-5] Computed (vs used) value of mix() functions

[fantasai]

The mix() family of functions compute a weighted average from a list of items and corresponding percentages.

Some mix() functions can be computed through to a final value, but others (e.g. calc-mix() combining percentages and lengths) cannot until used value time.

When they cannot be computed to a single value, should these functions:

• Simplify as much as possible given the information available, potentially collapsing some items in the list but not others?
• Or simplify each argument individually, preserving the number of items in the list?

Tab and I are currently specifying the latter option, but wanted to hear from others.