Serialize a color-mix() still assumes that 1) the <color-interpolation-method> is required and always serialized, 2) there isn't a <hue-interpolation-method>, and 3) that there are only two colors.

WPT expects the interpolation method to be omitted if it's "in oklab", which matches the usual shortest-serialization rule. It doesn't have coverage for the hue interpolation method or variable numbers of colors.