I filed a related issue for dynamic-range-limit-mix(), #11678, but the same issue exists for color-mix().

In the "percent normalization" algorithm, https://drafts.csswg.org/css-color-5/#color-mix-percent-norm, step 5 states:

5. If the percentages sum to zero, the function is invalid.

What should happen in the case that the percentages are calc() values? That is, what does it mean for the function to be invalid at style building / computed-value time?