@csstools/postcss-progressive-custom-properties

PostCSS Progressive Custom Properties

PostCSS Progressive Custom Properties is a utility plugin to correctly declare Custom Property fallbacks and enhancements.

[!WARNING] It is not intended to be used directly by stylesheet authors. Meant to be included in other PostCSS plugins that provide CSS value transforms as fallbacks.

Custom Properties are not discarded like regular declarations when invalid. This makes it tricky to provide fallback values for older browsers.

The solution is to wrap Custom Property declarations in an @supports rule.

:root {
	/* fallback */
	--a-color: red;
	/* progressive enhancement */
	--a-color: oklch(40% 0.234 0.39 / var(--opacity-50));
}

/* becomes */

:root {
	--a-color: red;
}

@supports (color: oklch(0% 0 0)) {
	:root {
		--a-color: oklch(40% 0.234 0.39 / var(--opacity-50));
	}
}

Ignored values

initial and <white space> are ignored.

.initial {
	--prop-1: red;
	--prop-1: initial;
}

.white-space {
	--prop-1: red;
	--prop-1:;

	--prop-2: red;
	--prop-2: ;

	--prop-3: red;
	--prop-3:    ;
}

/* remains */

.initial {
	--prop-1: red;
	--prop-1: initial;
}

.white-space {
	--prop-1: red;
	--prop-1:;

	--prop-2: red;
	--prop-2: ;

	--prop-3: red;
	--prop-3:    ;
}

Usage

Add PostCSS Progressive Custom Properties to your project:

npm install @csstools/postcss-progressive-custom-properties --save-dev

Use PostCSS Progressive Custom Properties as a PostCSS plugin:

const postcss = require('postcss');
const postcssCustomProperties = require('@csstools/postcss-progressive-custom-properties');

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

@supports

This plugin wraps Custom Property override declarations in an @supports rule. With PostCSS 8 this trigger declaration visitors to run again.

Make sure your plugin detects and ignores values inside relevant @supports rules.

Reading list

• https://moderncss.dev/how-custom-property-values-are-computed/

PostCSS Progressive Custom Properties runs in all Node environments, with special instructions for:

Node	PostCSS CLI	Webpack	Gulp	Grunt

Similar packages

Other packages similar to @csstools/postcss-progressive-custom-properties

postcss-custom-properties

postcss-custom-properties is a PostCSS plugin that transforms W3C CSS Custom Properties for cascading variables. It provides similar functionality to @csstools/postcss-progressive-custom-properties by enabling the use of custom properties with fallbacks for older browsers. However, it may not offer the same level of progressive enhancement.

postcss-preset-env

postcss-preset-env is a PostCSS plugin that allows you to use future CSS features today. It includes support for custom properties and provides fallbacks for older browsers. Compared to @csstools/postcss-progressive-custom-properties, it offers a broader range of CSS features beyond just custom properties.