The CSS Spec has been updated to handle missing channels in the relative-color syntax, using the same logic as interpolation: w3c/csswg-drafts#7771

In the current proposal, we define that behavior for interpolations - but not for conversion, and therefor also not for channel adjustments. Since CSS doesn't have 'conversion' as a user-facing feature outside interpolation/relative-colors, I think it's now safe to consider this part of the conversion algorithm, and implement it anywhere color spaces are converted.

(As part of this, we should also make sure to track edge cases like conversion within a space. Since the conversion logic explicitly marks powerless channels as 'missing' - a conversion of color.to-space(hsl(0 0% 0%)) should return hsl(none 0% 0%))