I recently learned that any calc() value can contain font/container/viewport relative lengths, as they can be used from with a sign() math function. One implication of this seems to be that eager evaluation of calc() at parse time for legacy colors can't always complete. For a concrete example, consider:

background-color: rgb(calc(90 + (90 * sign(1vw + 2em))), 0, 0);

I don't believe that has a value until style resolution since we don't know if the expression 1vw + 2em will be positive or negative.

My previous understanding of the rules for sRGB color types (rgb/hsl/hwb) is that they serialize without calc() at all levels (specified, computed, etc), but I am not sure what to do when the calc can't be fully resolved at specified value time.

The language in https://drafts.csswg.org/css-color-4/#resolving-sRGB-values I think actually allows serializing the calc() in these cases, since it uses the phrasing: "[...] when calc() in sRGB colors resolves to a single value [...]", but I wanted to make sure that was the intention.