252 lines
5.3 KiB
Markdown
252 lines
5.3 KiB
Markdown
# PostCSS Is Pseudo [<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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[![CSS Standard Status][css-img]][css-url]
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[<img alt="Build Status" src="https://github.com/csstools/postcss-plugins/workflows/test/badge.svg" height="20">][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 Is Pseudo Class] lets you use the `:is` pseudo class function, following the
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[CSS Selector] specification.
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```pcss
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:is(input, button):is(:hover, :focus) {
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order: 1;
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}
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```
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Becomes :
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```pcss
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input:hover {
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order: 1;
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}
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input:focus {
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order: 1;
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}
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button:hover {
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order: 1;
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}
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button:focus {
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order: 1;
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}
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```
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## Usage
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Add [PostCSS Is Pseudo Class] to your project:
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```bash
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npm install @csstools/postcss-is-pseudo-class --save-dev
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```
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Use [PostCSS Is Pseudo Class] as a [PostCSS] plugin:
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```js
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import postcss from 'postcss';
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import postcssIsPseudoClass from '@csstools/postcss-is-pseudo-class';
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postcss([
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postcssIsPseudoClass(/* pluginOptions */)
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]).process(YOUR_CSS /*, processOptions */);
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```
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[PostCSS Is Pseudo Class] runs in all Node environments, with special instructions for:
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| [Node](INSTALL.md#node) | [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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## Options
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### preserve
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The `preserve` option determines whether the original notation
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is preserved. By default, it is not preserved.
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```js
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postcss([
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postcssIsPseudoClass({ preserve: true })
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]).process(YOUR_CSS /*, processOptions */);
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```
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```pcss
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:is(input, button):is(:hover, :focus) {
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order: 1;
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}
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```
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Becomes :
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```pcss
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input:hover {
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order: 1;
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}
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input:focus {
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order: 1;
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}
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button:hover {
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order: 1;
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}
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button:focus {
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order: 1;
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}
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:is(input, button):is(:hover, :focus) {
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order: 1;
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}
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```
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### specificityMatchingName
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The `specificityMatchingName` option allows you to change to selector used to adjust specificity.
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The default value is `does-not-exist`.
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If this is an actual class, id or tag name in your code, you will need to set a different option here.
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See how `:not` is used to modify [specificity](#specificity).
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```js
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postcss([
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postcssIsPseudoClass({ specificityMatchingName: 'something-random' })
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]).process(YOUR_CSS /*, processOptions */);
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```
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```pcss
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:is(.button, button):hover {
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order: 7;
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}
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```
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Becomes :
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```pcss
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.button:hover {
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order: 7;
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}
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button:not(.something-random):hover {
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order: 7;
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}
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```
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### onComplexSelector
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Warn on complex selectors in `:is` pseudo class functions.
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```js
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postcss([
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postcssIsPseudoClass({ onComplexSelector: 'warning' })
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]).process(YOUR_CSS /*, processOptions */);
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```
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### onPseudoElement
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Warn when pseudo elements are used in `:is` pseudo class functions.
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⚠️ Pseudo elements are always invalid and will be transformed to `::-csstools-invalid-<pseudo-name>`.
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```js
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postcss([
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postcssIsPseudoClass({ onPseudoElement: 'warning' })
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]).process(YOUR_CSS /*, processOptions */);
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```
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```css
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:is(::after):hover {
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order: 1.0;
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}
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/* becomes */
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::-csstools-invalid-after:hover {
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order: 1.0;
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}
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```
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## ⚠️ Known shortcomings
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### Specificity
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`:is` takes the specificity of the most specific list item.
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We can increase specificity with `:not` selectors, but we can't decrease it.
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Converted selectors are ensured to have the same specificity as `:is` for the most important bit.
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Less important bits can have higher specificity that `:is`.
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Before :
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[specificity: 0, 2, 0](https://polypane.app/css-specificity-calculator/#selector=%3Ais(%3Ahover%2C%20%3Afocus)%3Ais(.button%2C%20button))
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```pcss
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:is(:hover, :focus):is(.button, button) {
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order: 7;
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}
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```
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After :
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```pcss
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/* specificity: [0, 2, 0] */
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.button:hover {
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order: 7;
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}
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/* specificity: [0, 2, 1] */
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/* last bit is higher than it should be, but middle bit matches */
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button:not(.does-not-exist):hover {
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order: 7;
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}
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/* specificity: [0, 2, 0] */
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.button:focus {
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order: 7;
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}
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/* specificity: [0, 2, 1] */
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/* last bit is higher than it should be, but middle bit matches */
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button:not(.does-not-exist):focus {
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order: 7;
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}
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```
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### Complex selectors
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Before :
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```pcss
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:is(.alpha > .beta) ~ :is(:focus > .beta) {
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order: 2;
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}
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```
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After :
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```pcss
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.alpha > .beta ~ :focus > .beta {
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order: 2;
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}
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```
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_this is a different selector than expected as `.beta ~ :focus` matches `.beta` followed by `:focus`._<br>
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_avoid these cases._<br>
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_writing the selector without `:is()` is advised here_
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```pcss
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/* without is */
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.alpha:focus > .beta ~ .beta {
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order: 2;
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}
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```
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If you have a specific pattern you can open an issue to discuss it.
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We can detect and transform some cases but can't generalize them into a single solution that tackles all of them.
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[cli-url]: https://github.com/csstools/postcss-plugins/actions/workflows/test.yml?query=workflow/test
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[css-img]: https://cssdb.org/images/badges/is-pseudo-class.svg
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[css-url]: https://cssdb.org/#is-pseudo-class
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[discord]: https://discord.gg/bUadyRwkJS
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[npm-img]: https://img.shields.io/npm/v/@csstools/postcss-is-pseudo-class.svg
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[npm-url]: https://www.npmjs.com/package/@csstools/postcss-is-pseudo-class
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[CSS Selector]: https://www.w3.org/TR/selectors-4/#matches
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[PostCSS]: https://github.com/postcss/postcss
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[PostCSS Is Pseudo Class]: https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-is-pseudo-class
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