https://drafts.csswg.org/css-color-5/#relative-color-function

css-color-5 allows us to describe components of custom color spaces via the components descriptor. However, we cannot have syntax that becomes invalid at parse time based on the value of a descriptor.
Therefore, we should allow any identifier to be used for RCS in color() at parse time, and make it IACVT if the wrong identifiers are used.

cc @tabatkins @svgeesus