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# PostCSS Progressive Custom Properties [<img src="https://postcss.github.io/postcss/logo.svg" alt="PostCSS" width="90" height="90" align="right">][postcss]
[![NPM Version][npm-img]][npm-url]
[![Build Status][cli-img]][cli-url]
[<img alt="Discord" src="https://shields.io/badge/Discord-5865F2?logo=discord&logoColor=white">][discord]
[PostCSS Progressive Custom Properties] is a utility plugin to correctly declare Custom Property fallbacks and enhancements.
⚠️ 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.](https://www.w3.org/TR/css-variables-1/#invalid-variables)
This makes it tricky to provide fallback values for older browsers.
The solution is to wrap Custom Property declarations in an `@supports` rule.
```pcss
: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.
```pcss
.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:
```bash
npm install @csstools/postcss-progressive-custom-properties --save-dev
```
Use [PostCSS Progressive Custom Properties] as a [PostCSS] plugin:
```js
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.
[PostCSS Progressive Custom Properties] runs in all Node environments, with special instructions for:
| [Node](INSTALL.md#node) | [PostCSS CLI](INSTALL.md#postcss-cli) | [Webpack](INSTALL.md#webpack) | [Create React App](INSTALL.md#create-react-app) | [Gulp](INSTALL.md#gulp) | [Grunt](INSTALL.md#grunt) |
| --- | --- | --- | --- | --- | --- |
[cli-img]: https://github.com/csstools/postcss-plugins/actions/workflows/test.yml/badge.svg
[cli-url]: https://github.com/csstools/postcss-plugins/actions/workflows/test.yml?query=workflow/test
[discord]: https://discord.gg/bUadyRwkJS
[npm-img]: https://img.shields.io/npm/v/@csstools/postcss-progressive-custom-properties.svg
[npm-url]: https://www.npmjs.com/package/@csstools/postcss-progressive-custom-properties
[PostCSS]: https://github.com/postcss/postcss
[PostCSS Progressive Custom Properties]: https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-progressive-custom-properties
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