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css-color-parser: preserve powerless hue in color-mix when interpolation space is the same as origin #1889

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Merged
romainmenke merged 2 commits into from
Aug 25, 2026
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css-color-parser: preserve powerless hue in color-mix when interpolation space is the same as origin #1889

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52b13c2
css-color-parser: preserve powerless hue in color-mix when interpolat…
romainmenke Aug 22, 2026
f08f759
cleanup and changelog
romainmenke Aug 25, 2026
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4 changes: 4 additions & 0 deletions packages/css-color-parser/CHANGELOG.md
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@@ -1,5 +1,9 @@
# Changes to CSS Color Parser

### Unreleased (patch)

- Preserve powerless hue in `color-mix()` when the input color space equals the interpolation color space, matching relative color syntax.

### 4.2.0

_August 15, 2026_
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2 changes: 1 addition & 1 deletion packages/css-color-parser/dist/index.mjs
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12 changes: 10 additions & 2 deletions packages/css-color-parser/src/color-data.ts
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Expand Up @@ -310,6 +310,14 @@ const predefinedRGB_or_XYZ_Spaces = new Set([
]);

export function colorDataToForInterpolation(colorData: ColorData, toNotation: ColorNotation): ColorData {
// https://drafts.csswg.org/css-color-4/#interpolation
// Steps 1-4 equate to a noop if the input color is already in the interpolation color space
if (colorData.colorNotation === toNotation) {
return {
...colorData,
};
}

// https://drafts.csswg.org/css-color-4/#interpolation
// 1. checking the two colors for analogous components and analogous sets which will be carried forward
const originalChannelValues = [...colorData.channels] as Color;
Expand All @@ -319,11 +327,11 @@ export function colorDataToForInterpolation(colorData: ColorData, toNotation: Co
};

// https://drafts.csswg.org/css-color-4/#interpolation
// 2. changing any powerless components to missing values
// 2. (if conversion is required) changing any powerless components to missing values
outputColorData.channels = convertPowerlessComponentsToMissingComponents(outputColorData.channels, colorData.colorNotation);

// https://drafts.csswg.org/css-color-4/#interpolation
// 3. converting them both to a given color space which will be referred to as the interpolation color space below.
// 3. (if required) converting them both to a given color space which will be referred to as the interpolation color space below.
//
// https://drafts.csswg.org/css-color-4/#color-conversion
// To convert a color...
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30 changes: 24 additions & 6 deletions packages/css-color-parser/src/functions/color-mix.ts
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Expand Up @@ -372,6 +372,8 @@ function colorMixRectangularPair(colorNotation: ColorNotation, a_color: ColorDat
const a_channels = colorDataToForInterpolation(a_color, colorNotation).channels;
const b_channels = colorDataToForInterpolation(b_color, colorNotation).channels;

// https://drafts.csswg.org/css-color-4/#interpolation
// 5. fill in missing components with the other color’s component values
a_channels[0] = fillInMissingComponent(a_channels[0], b_channels[0]);
b_channels[0] = fillInMissingComponent(b_channels[0], a_channels[0]);

Expand All @@ -381,6 +383,12 @@ function colorMixRectangularPair(colorNotation: ColorNotation, a_color: ColorDat
a_channels[2] = fillInMissingComponent(a_channels[2], b_channels[2]);
b_channels[2] = fillInMissingComponent(b_channels[2], a_channels[2]);

// https://drafts.csswg.org/css-color-4/#interpolation
// 6. (if required) fixing up the hues, depending on the selected <hue-interpolation-method>
// -> not required

// https://drafts.csswg.org/css-color-4/#interpolation
// 7. changing the color components to premultiplied form
a_channels[0] = premultiply(a_channels[0], a_alpha);
a_channels[1] = premultiply(a_channels[1], a_alpha);
a_channels[2] = premultiply(a_channels[2], a_alpha);
Expand All @@ -389,6 +397,9 @@ function colorMixRectangularPair(colorNotation: ColorNotation, a_color: ColorDat
b_channels[1] = premultiply(b_channels[1], b_alpha);
b_channels[2] = premultiply(b_channels[2], b_alpha);

// https://drafts.csswg.org/css-color-4/#interpolation
// 8. linearly interpolating each component of the computed value of the color separately
// 9. undoing premultiplication
const alpha = interpolate(a_alpha, b_alpha, progress);
const outputChannels: Color = [
un_premultiply(interpolate(a_channels[0], b_channels[0], progress), alpha),
Expand Down Expand Up @@ -547,6 +558,8 @@ function colorMixPolarPair(colorNotation: ColorNotation, hueInterpolationMethod:
a_alpha = Number.isNaN(a_alpha) ? b_alpha : a_alpha;
b_alpha = Number.isNaN(b_alpha) ? a_alpha : b_alpha;

// https://drafts.csswg.org/css-color-4/#interpolation
// 5. fill in missing components with the other color’s component values
const a_channels = colorDataToForInterpolation(a_color, colorNotation).channels;
const b_channels = colorDataToForInterpolation(b_color, colorNotation).channels;

Expand Down Expand Up @@ -579,6 +592,8 @@ function colorMixPolarPair(colorNotation: ColorNotation, hueInterpolationMethod:
break;
}

// https://drafts.csswg.org/css-color-4/#interpolation
// 5. fill in missing components with the other color’s component values
a_first = fillInMissingComponent(a_first, b_first);
b_first = fillInMissingComponent(b_first, a_first);

Expand All @@ -587,15 +602,12 @@ function colorMixPolarPair(colorNotation: ColorNotation, hueInterpolationMethod:

a_hue = fillInMissingComponent(a_hue, b_hue);
b_hue = fillInMissingComponent(b_hue, a_hue);

// https://drafts.csswg.org/css-color-4/#interpolation
// 6. (if required) fixing up the hues, depending on the selected <hue-interpolation-method>
if (Number.isNaN(a_hue) && Number.isNaN(b_hue)) {
// noop
} else {
if (Number.isNaN(a_hue)) {
a_hue = 0;
} else if (Number.isNaN(b_hue)) {
b_hue = 0;
}

const angleDiff = b_hue - a_hue;

switch (hueInterpolationMethod) {
Expand Down Expand Up @@ -634,11 +646,17 @@ function colorMixPolarPair(colorNotation: ColorNotation, hueInterpolationMethod:
}
}

// https://drafts.csswg.org/css-color-4/#interpolation
// 7. changing the color components to premultiplied form
a_first = premultiply(a_first, a_alpha);
a_second = premultiply(a_second, a_alpha);
b_first = premultiply(b_first, b_alpha);
b_second = premultiply(b_second, b_alpha);


// https://drafts.csswg.org/css-color-4/#interpolation
// 8. linearly interpolating each component of the computed value of the color separately
// 9. undoing premultiplication
let outputChannels: Color = [0, 0, 0];
const alpha = interpolate(a_alpha, b_alpha, progress);

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4 changes: 2 additions & 2 deletions packages/css-color-parser/test/basic/color-mix-function-oklch.mjs
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Expand Up @@ -4,12 +4,12 @@ import { parse } from '../util/parse.mjs';
import { serialize_OKLCH_data } from '../util/serialize.mjs';

const tests = [
['color-mix(in oklch, oklch(100% 0% 60deg), oklch(50% 50% 0deg))', 'oklch(0.75 0.1 0)'],
['color-mix(in oklch, oklch(100% 0% 60deg), oklch(50% 50% 0deg))', 'oklch(0.75 0.1 30)'],
['color-mix(in oklch, oklch(100% 0% none), oklch(50% 50% 0deg))', 'oklch(0.75 0.1 0)'],
['color-mix(in oklch, rgb(255, 255, 255), rgb(180, 6, 95))', 'oklch(0.75031 0.10016 359.858)'],
['color-mix(in lch, oklch(75% 0% 60deg), oklch(75% 50% 0deg))', 'oklch(0.74979 0.09824 0.10588)'],
['color-mix(in oklch, oklch(100% 0% none), oklch(50% 50% 0deg))', 'oklch(0.75 0.1 0)'],
['color-mix(in oklch, oklch(100% none 60deg), oklch(50% 50% 0deg))', 'oklch(0.75 0.2 0)'],
['color-mix(in oklch, oklch(100% none 60deg), oklch(50% 50% 0deg))', 'oklch(0.75 0.2 30)'],

// Analogous sets
['color-mix(in oklch, rgb(none none none), oklch(0.5 0.2 50))', 'oklch(0.5 0.2 50)'],
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10 changes: 5 additions & 5 deletions packages/css-color-parser/test/basic/color-mix-function.mjs
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Expand Up @@ -115,9 +115,9 @@ const tests = [
['color-mix(in hsl decreasing hue, hsl(180deg 50% 50%), hsl(180deg 50% 50%))', canonicalize('hsl(180deg 50% 50%)')],

['color-mix(in hsl longer hue, hsl(50deg 0% 50%), hsl(50deg 0% 50%))', canonicalize('hsl(230deg 0% 50%)')],
['color-mix(in hsl, color-mix(in hsl longer hue, hsl(50deg 0% 50%), hsl(50deg 0% 50%)), hsl(180deg 100% 50%))', canonicalize('hsl(180deg 50% 50%)')],
['color-mix(in hsl, color-mix(in hsl longer hue, hsl(50deg 0% 50%), hsl(50deg 0% 50%)), hsl(180deg 100% 50%))', canonicalize('hsl(205deg 50% 50%)')],

['color-mix(in hsl, hsl(30deg 40% 80% / 25%) 0%, hsl(90deg none none / none))', canonicalize('hsl(30deg 40% 80% / 25%)')],
['color-mix(in hsl, hsl(30deg 40% 80% / 25%) 0%, hsl(90deg none none / none))', canonicalize('hsl(90deg 40% 80% / 25%)')],
['color-mix(in hwb, hwb(30deg 30% 40% / 25%) 0%, hwb(90deg none none / 0.5))', canonicalize('hwb(90deg 30% 40% / 0.5)')],
['color-mix(in hsl, hsl(from hsl(none 50% 50%) h s l), hsl(from hsl(120deg 50% 50%) h s l))', canonicalize('hsl(120deg 50% 50%)')],
['color-mix(in hsl, hsl(from hsl(0deg 50% 50%) none s l), hsl(from hsl(120deg 50% 50%) h s l))', canonicalize('hsl(120deg 50% 50%)')],
Expand Down Expand Up @@ -193,7 +193,7 @@ assert.deepStrictEqual(
color(parse('color-mix(in lch, lch(50% 50% 180deg), lch(100% 0% 0deg))')),
{
colorNotation: 'lch',
channels: [75, 37.5, 180],
channels: [75, 37.5, 90],
alpha: 1,
syntaxFlags: new Set(['color-mix']),
},
Expand Down Expand Up @@ -233,7 +233,7 @@ assert.deepStrictEqual(
color(parse('color-mix(in lch, lch(100 0 40deg), lch(100 0 60deg))')),
{
colorNotation: 'lch',
channels: [100, 0, Number.NaN],
channels: [100, 0, 50],
alpha: 1,
syntaxFlags: new Set(['color-mix']),
},
Expand Down Expand Up @@ -293,7 +293,7 @@ assert.deepStrictEqual(
color(parse('color-mix(in hsl, lch(none 0 30), hsl(50 0 none))')),
{
colorNotation: 'hsl',
channels: [Number.NaN, 0, Number.NaN],
channels: [50, 0, Number.NaN],
alpha: 1,
syntaxFlags: new Set(['color-mix']),
},
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14 changes: 7 additions & 7 deletions packages/css-color-parser/test/basic/none-exhaustive.mjs
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Expand Up @@ -14,7 +14,7 @@ import { parse } from '../util/parse.mjs';
// https://github.com/w3c/csswg-drafts/issues/14100
['lch(from orchid l 0 h)', 'lch(62.75256542 0 326.96909222)'],
['lch(from lch(from orchid l 0 h) l c h)', 'lch(62.75256542 0 326.96909222)'],
['color-mix(in lch, lch(from orchid l 0 h))', 'lch(62.75256542 0 none)'],
['color-mix(in lch, lch(from orchid l 0 h))', 'lch(62.75256542 0 326.96909222)'],

// exhaustive tests
// HSL - RCS
Expand Down Expand Up @@ -71,10 +71,10 @@ import { parse } from '../util/parse.mjs';

// HSL - color-mix
['color-mix(in hsl, hsl(none 50% 50%), hsl(11 33 44))', 'color(srgb 0.66505 0.34646833 0.27495)', 'hsl(11 41.5% 47%)'],
['color-mix(in hsl, hsl(180 none 50%), hsl(11 33 44))', 'color(srgb 0.6251 0.37177 0.3149)', 'hsl(11 33% 47%)'],
['color-mix(in hsl, hsl(180 none 50%), hsl(11 33 44))', 'color(srgb 0.441565 0.6251 0.3149)', 'hsl(95.5 33% 47%)'],
['color-mix(in hsl, hsl(180 50% none), hsl(11 33 44))', 'color(srgb 0.40652333 0.6226 0.2574)', 'hsl(95.5 41.5% 44%)'],
['color-mix(in hsl, hsl(none none 50%), hsl(11 33 44))', 'color(srgb 0.6251 0.37177 0.3149)', 'hsl(11 33% 47%)'],
['color-mix(in hsl, hsl(180 none none), hsl(11 33 44))', 'color(srgb 0.5852 0.34804 0.2948)', 'hsl(11 33% 44%)'],
['color-mix(in hsl, hsl(180 none none), hsl(11 33 44))', 'color(srgb 0.41338 0.5852 0.2948)', 'hsl(95.5 33% 44%)'],
['color-mix(in hsl, hsl(none 50% none), hsl(11 33 44))', 'color(srgb 0.6226 0.32435333 0.2574)', 'hsl(11 41.5% 44%)'],
['color-mix(in hsl, hsl(none none none), hsl(11 33 44))', 'color(srgb 0.5852 0.34804 0.2948)', 'hsl(11 33% 44%)'],

Expand Down Expand Up @@ -295,9 +295,9 @@ import { parse } from '../util/parse.mjs';
['color-mix(in lch, hwb(none none none), lch(11 33 44))', 'lch(11 33 44)'],

['color-mix(in lch, lch(none 20 180), lch(11 33 44))', 'lch(11 26.5 112)'],
['color-mix(in lch, lch(20 none 180), lch(11 33 44))', 'lch(15.5 33 44)'],
['color-mix(in lch, lch(20 none 180), lch(11 33 44))', 'lch(15.5 33 112)'],
['color-mix(in lch, lch(20 20 none), lch(11 33 44))', 'lch(15.5 26.5 44)'],
['color-mix(in lch, lch(none none 180), lch(11 33 44))', 'lch(11 33 44)'],
['color-mix(in lch, lch(none none 180), lch(11 33 44))', 'lch(11 33 112)'],
['color-mix(in lch, lch(20 none none), lch(11 33 44))', 'lch(15.5 33 44)'],
['color-mix(in lch, lch(none 20 none), lch(11 33 44))', 'lch(11 26.5 44)'],
['color-mix(in lch, lch(none none none), lch(11 33 44))', 'lch(11 33 44)'],
Expand Down Expand Up @@ -407,9 +407,9 @@ import { parse } from '../util/parse.mjs';
['color-mix(in oklch, lch(none none none), oklch(0.11 0.33 44))', 'oklch(0.11 0.33 44)'],

['color-mix(in oklch, oklch(none 0.2 180), oklch(0.11 0.33 44))', 'oklch(0.11 0.265 112)'],
['color-mix(in oklch, oklch(0.2 none 180), oklch(0.11 0.33 44))', 'oklch(0.155 0.33 44)'],
['color-mix(in oklch, oklch(0.2 none 180), oklch(0.11 0.33 44))', 'oklch(0.155 0.33 112)'],
['color-mix(in oklch, oklch(0.2 0.2 none), oklch(0.11 0.33 44))', 'oklch(0.155 0.265 44)'],
['color-mix(in oklch, oklch(none none 180), oklch(0.11 0.33 44))', 'oklch(0.11 0.33 44)'],
['color-mix(in oklch, oklch(none none 180), oklch(0.11 0.33 44))', 'oklch(0.11 0.33 112)'],
['color-mix(in oklch, oklch(0.2 none none), oklch(0.11 0.33 44))', 'oklch(0.155 0.33 44)'],
['color-mix(in oklch, oklch(none 0.2 none), oklch(0.11 0.33 44))', 'oklch(0.11 0.265 44)'],
['color-mix(in oklch, oklch(none none none), oklch(0.11 0.33 44))', 'oklch(0.11 0.33 44)'],
Expand Down
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