[css-color-5] `color-mix()` serialization is outdated

[AtkinsSJ]

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.

[yisibl]

I'd like to know how to serialize multiple colors when they are repeated?

e.g.

• color-mix(green, green, green)
• color-mix(red 100%, red 100%)
• color-mix(currentcolor, currentcolor)

[css-meeting-bot]

The CSS Working Group just discussed [css-color-5] `color-mix()` serialization is outdated, and agreed to the following:

• RESOLVED: Require serializing without color space if it matches the default (oklab)
• RESOLVED: Multiple identical entries do not collapse.
• RESOLVED: Colors are not collapsed
• RESOLVED: If percentages for N arguments are 1/N (and are therefore all omittable), omit them.
• RESOLVED: If percentages for N arguments are 1/N (and are therefore all omittable), omit them. Otherwise serialize the percentages (even if originally omitted).
• ACTION: update spec to handle hue-interpolation method

The full IRC log of that discussion

<emilio> ChrisL: it is
<emilio> ... I had some problems with how to update it
<emilio> ... it used to be the case that you had two colors and if you don't specify them it's 50% and 50%
<emilio> ... so with more colors I'd expect 100% / N
<fantasai> emilio: Some implementation updates already, so can check what happens
<emilio> Rossen: is there any other sensible alternative?
<Rossen> ack fantasai
<emilio> fantasai: some questions about color-interpolation-method being optional
<emilio> ... we are probably happy to make that change?
<emilio> ChrisL: we already resolved on it, but yeah it should serialize to the shortest value (omit oklab if needed)
<emilio> PROPOSED: Allow to serialize without color space if it matches the default (oklab)
<emilio> s/Allow to serialize/Require serializing
<emilio> emilio: potential compat is `color-mix(in oklab,` now serializes without
<emilio> ... but probably fine
<emilio> fantasai: another one would be do we ever plan to change the default
<emilio> emilio: Even if we changed it we would potentially be breaking stuff so I'd rather do the simple thing and omit oklab
<emilio> PROPOSED: Require serializing without color space if it matches the default (oklab)
<emilio> RESOLVED: Require serializing without color space if it matches the default (oklab)
<emilio> ChrisL: other question is serializing with identical colors, like `color-mix(green, green, green, red)`, so you need to preserve them
<emilio> TabAtkins: you could do `green 75%, red 25%)`
<fantasai> PROPOSED: Multiple identical entries do not collapse.
<fantasai> RESOLVED: Multiple identical entries do not collapse.
<kbabbitt> +1
<emilio> RESOLVED: Colors are not collapsed
<emilio> fantasai: if you have both percentages 50% do you serialize them?
<emilio> ChrisL: the current algorithm omits the percentages
<fantasai> PROPOSED: If percentages for N arguments are 1/N (and are therefore all omittable), omit them.
<emilio> fantasai: if I write `red 50%, green 25%, blue 25%`, how do we serialize that out?
<fantasai> color-mix(red 50%, green 25%, blue 25%)
<fantasai> color-mix(red 50%, green, blue)
<fantasai> color-mix(red, green 25%, blue 25%)
<lea> q?
<lea> q+
<emilio> fantasai: we don't know what you meant, I suggest we drop the percents if all drop
<emilio> ChrisL: the current text says "if both are 50% you drop them, otherwise you don't"
<emilio> ... I'd like to preserve that, only drop if all are omittable
<Rossen> ack lea
<emilio> lea: we do have this rule
<emilio> ... about shortest serialization
<emilio> ... could be useful?
<emilio> emilio: within reason :)
<emilio> fantasai: if you have four colors there's no single answer
<ChrisL> as simple as possible. but no simpler
<lea> `red 30%, green 30%, yellow 20%, purple 20%`
<fantasai> PROPOSED: If percentages for N arguments are 1/N (and are therefore all omittable), omit them. Otherwise serialize the percentages (even if originally omitted).
<lea> +1
<emilio> RESOLVED: If percentages for N arguments are 1/N (and are therefore all omittable), omit them.
<fantasai> RESOLVED: If percentages for N arguments are 1/N (and are therefore all omittable), omit them. Otherwise serialize the percentages (even if originally omitted).
<fantasai> ACTION: update spec to handle hue-interpolation method