@csstools/postcss-alpha-function

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.color {
	color: alpha(from #dddd / calc(alpha / 2));
}

:root {
	--a-color: alpha(from rgb(2 1 0 / var(--a)) / calc(alpha / 2));
}

/* becomes */

.color {
	color: rgb(from #dddd r g b / calc(alpha / 2));
}

:root {
	--a-color: rgb(from rgb(2 1 0 / var(--a)) r g b / calc(alpha / 2));
}

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npm install postcss @csstools/postcss-alpha-function --save-dev

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const postcss = require('postcss');
const postcssAlphaFunction = require('@csstools/postcss-alpha-function');

postcss([
	postcssAlphaFunction(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

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postcssAlphaFunction({ preserve: true })

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.color {
	color: alpha(from #dddd / calc(alpha / 2));
}

:root {
	--a-color: alpha(from rgb(2 1 0 / var(--a)) / calc(alpha / 2));
}

/* becomes */

.color {
	color: rgb(from #dddd r g b / calc(alpha / 2));
	color: alpha(from #dddd / calc(alpha / 2));
}

:root {
	--a-color: rgb(from rgb(2 1 0 / var(--a)) r g b / calc(alpha / 2));
}

@supports (color: alpha(from red / 1)) and (color: rgb(0 0 0 / 0)) {
:root {
	--a-color: alpha(from rgb(2 1 0 / var(--a)) / calc(alpha / 2));
}
}

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postcssAlphaFunction({ enableProgressiveCustomProperties: false })

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.color {
	color: alpha(from #dddd / calc(alpha / 2));
}

:root {
	--a-color: alpha(from rgb(2 1 0 / var(--a)) / calc(alpha / 2));
}

/* becomes */

.color {
	color: rgb(from #dddd r g b / calc(alpha / 2));
	color: alpha(from #dddd / calc(alpha / 2));
}

:root {
	--a-color: rgb(from rgb(2 1 0 / var(--a)) r g b / calc(alpha / 2));
	--a-color: alpha(from rgb(2 1 0 / var(--a)) / calc(alpha / 2));
}