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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] |
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[![NPM Version][npm-img]][npm-url] |
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[![Build Status][cli-img]][cli-url] |
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[<img alt="Discord" src="https://shields.io/badge/Discord-5865F2?logo=discord&logoColor=white">][discord] |
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[PostCSS Progressive Custom Properties] is a utility plugin to correctly declare Custom Property fallbacks and enhancements. |
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⚠️ It is not intended to be used directly by stylesheet authors. |
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Meant to be included in other PostCSS plugins that provide CSS value transforms as fallbacks. |
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[Custom Properties are not discarded like regular declarations when invalid.](https://www.w3.org/TR/css-variables-1/#invalid-variables) |
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This makes it tricky to provide fallback values for older browsers. |
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The solution is to wrap Custom Property declarations in an `@supports` rule. |
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```pcss |
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:root { |
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/* fallback */ |
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--a-color: red; |
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/* progressive enhancement */ |
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--a-color: oklch(40% 0.234 0.39 / var(--opacity-50)); |
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} |
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/* becomes */ |
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:root { |
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--a-color: red; |
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} |
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@supports (color: oklch(0% 0 0)) { |
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:root { |
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--a-color: oklch(40% 0.234 0.39 / var(--opacity-50)); |
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} |
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} |
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``` |
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## Ignored values |
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`initial` and `<white space>` are ignored. |
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```pcss |
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.initial { |
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--prop-1: red; |
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--prop-1: initial; |
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} |
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.white-space { |
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--prop-1: red; |
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--prop-1:; |
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--prop-2: red; |
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--prop-2: ; |
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--prop-3: red; |
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--prop-3: ; |
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} |
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/* remains */ |
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.initial { |
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--prop-1: red; |
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--prop-1: initial; |
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} |
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.white-space { |
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--prop-1: red; |
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--prop-1:; |
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--prop-2: red; |
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--prop-2: ; |
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--prop-3: red; |
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--prop-3: ; |
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} |
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``` |
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## Usage |
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Add [PostCSS Progressive Custom Properties] to your project: |
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```bash |
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npm install @csstools/postcss-progressive-custom-properties --save-dev |
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``` |
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Use [PostCSS Progressive Custom Properties] as a [PostCSS] plugin: |
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```js |
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const postcss = require('postcss'); |
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const postcssCustomProperties = require('@csstools/postcss-progressive-custom-properties'); |
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postcss([ |
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postcssProgressiveCustomProperties() |
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]).process(YOUR_CSS /*, processOptions */); |
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``` |
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## @supports |
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This plugin wraps Custom Property override declarations in an `@supports` rule. |
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With PostCSS 8 this trigger declaration visitors to run again. |
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Make sure your plugin detects and ignores values inside relevant `@supports` rules. |
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[PostCSS Progressive Custom Properties] runs in all Node environments, with special instructions for: |
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| [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) | |
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| --- | --- | --- | --- | --- | --- | |
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[cli-img]: https://github.com/csstools/postcss-plugins/actions/workflows/test.yml/badge.svg |
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[cli-url]: https://github.com/csstools/postcss-plugins/actions/workflows/test.yml?query=workflow/test |
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[discord]: https://discord.gg/bUadyRwkJS |
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[npm-img]: https://img.shields.io/npm/v/@csstools/postcss-progressive-custom-properties.svg |
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[npm-url]: https://www.npmjs.com/package/@csstools/postcss-progressive-custom-properties |
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[PostCSS]: https://github.com/postcss/postcss |
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[PostCSS Progressive Custom Properties]: https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-progressive-custom-properties
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