/postcss

Transforming CSS with JS plugins

Primary LanguageJavaScriptMIT LicenseMIT

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PostCSS is a tool for transforming CSS with JS plugins. These plugins can support variables and mixins, transpile future CSS syntax, inline images, and more.

PostCSS is used by industry leaders including Google, Twitter, Alibaba, and Shopify. The Autoprefixer PostCSS plugin is one of the most popular CSS processors.

PostCSS can do the same work as preprocessors like Sass, Less, and Stylus. But PostCSS is modular, 3-30 times faster, and much more powerful.

Twitter account: @postcss. VK.com page: postcss.

Examples Features Usage Plugins Write Your Own Plugin Options
Sponsored by Evil Martians

What is PostCSS

PostCSS itself is very small. It includes only a CSS parser, a CSS node tree API, a source map generator, and a node tree stringifier.

All CSS transformations are made by plugins. And these plugins are just small plain JS functions, which receive a CSS node tree, transform it, and return a modified tree.

You can use the cssnext plugin pack and write future CSS code right now:

:root {
    --mainColor: #ffbbaaff;
}
@custom-media    --mobile (width <= 640px);
@custom-selector --heading h1, h2, h3, h4, h5, h6;

.post-article :--heading {
    color: color( var(--mainColor) blackness(+20%) );
}
@media (--mobile) {
    .post-article :--heading {
        margin-top: 0;
    }
}

Or if you like the Sass syntax, you could use PreCSS plugin pack:

@define-mixin social-icon $network $color {
    &.is-$network {
        background: $color;
    }
}

.social-icon {
    @mixin social-icon twitter  #55acee;
    @mixin social-icon facebook #3b5998;

    padding: 10px 5px;
    @media (max-width: 640px) {
        padding: 0;
    }
}

Features

Preprocessors are template languages, where you mix styles with code (like PHP does with HTML).

In contrast, in PostCSS you write a custom subset of CSS. All code can only be in JS plugins.

As a result, PostCSS offers three main benefits:

  • Performance: PostCSS, written in JS, is 3 times faster than libsass, which is written in C++.
  • Future CSS: PostCSS plugins can read and rebuild an entire document, meaning that they can provide new language features. For example, cssnext transpiles the latest W3C drafts to current CSS syntax.
  • New abilities: PostCSS plugins can read and change every part of CSS. It makes many new classes of tools possible. Autoprefixer, rtlcss, doiuse or postcss-colorblind are good examples.

Usage

Start using PostCSS in just two steps:

  1. Add PostCSS to your build tool.
  2. Select plugins from the list below and add them to your PostCSS process.

There are plugins for Grunt, Gulp, webpack, Broccoli, Brunch and ENB.

gulp.task('css', function () {
    var postcss = require('gulp-postcss');
    return gulp.src('src/**/*.css')
        .pipe( postcss([ require('cssnext')(), require('cssnano')() ]) )
        .pipe( gulp.dest('build/') );
});

For other environments, you can use the CLI tool or the JS API:

var postcss = require('postcss');
postcss([ require('cssnext')(), require('cssnano')() ])
    .process(css, { from: 'src/app.css', to: 'app.css' })
    .then(function (result) {
        fs.writeFileSync('app.css', result.css);
        if ( result.map ) fs.writeFileSync('app.css.map', result.map);
    });

You can also use PostCSS plugins with Stylus by using poststylus.

Read the PostCSS API for more details about the JS API.

Plugins

Go to postcss.parts for a searchable catalog of the plugins mentioned below.

Control

There are two ways to make PostCSS magic more explicit.

Limit a plugin's local stylesheet context using postcss-plugin-context:

.css-example.is-test-for-css4-browsers {
    color: gray(255, 50%);
}
@context cssnext {
    .css-example.is-fallback-for-all-browsers {
        color: gray(255, 50%);
    }
}

Or enable plugins directly in CSS using postcss-use:

@use autoprefixer(browsers: ['last 2 versions']);

:fullscreen a {
    display: flex
}

Packs

  • atcss contains plugins that transform your CSS according to special annotation comments.
  • cssnano contains plugins that optimize CSS size for use in production.
  • cssnext contains plugins that allow you to use future CSS features today.
  • precss contains plugins that allow you to use Sass-like CSS.
  • rucksack contains plugins to speed up CSS development with new features and shortcuts.
  • stylelint contains plugins that lint your stylesheets.

Future CSS Syntax

See also cssnext plugins pack to add future CSS syntax by one line of code.

Fallbacks

Language Extensions

See also precss plugins pack to add them by one line of code.

Colors

Images and Fonts

Grids

  • postcss-grid adds a semantic grid system.
  • postcss-neat is a semantic and fluid grid framework.
  • lost feature-rich calc() grid system by Jeet author.

Optimizations

  • postcss-calc reduces calc() to values (when expressions involve the same units).
  • postcss-import inlines the stylesheets referred to by @import rules.
  • postcss-single-charset ensures that there is one and only one @charset rule at the top of file.
  • postcss-zindex rebases positive z-index values.
  • css-byebye removes the CSS rules that you don’t want.
  • css-mqpacker joins matching CSS media queries into a single statement.
  • stylehacks removes CSS hacks based on browser support.

See also plugins in modular minifier cssnano.

Shortcuts

Others

Analysis

  • postcss-bem-linter lints CSS for conformance to SUIT CSS methodology.
  • postcss-cssstats returns an object with CSS statistics.
  • css2modernizr creates a Modernizr config file that requires only the tests that your CSS uses.
  • doiuse lints CSS for browser support, using data from Can I Use.
  • immutable-css lints CSS for class mutations.
  • list-selectors lists and categorizes the selectors used in your CSS, for code review.

Reporters

Fun

How to Develop a PostCSS Plugin

Options

Source Map

PostCSS has great source maps support. It can read and interpret maps from previous transformation steps, autodetect the format that you expect, and output both external and inline maps.

To ensure that you generate an accurate source map, you must indicate the input and output CSS file paths — using the options from and to, respectively.

To generate a new source map with the default options, simply set map: true. This will generate an inline source map that contains the source content. If you don’t want the map inlined, you can set map.inline: false.

processor
    .process(css, {
        from: 'app.sass.css',
        to:   'app.css',
        map: { inline: false },
    })
    .then(function (result) {
        result.map //=> '{ "version":3,
                   //      "file":"app.css",
                   //      "sources":["app.sass"],
                   //       "mappings":"AAAA,KAAI" }'
    });

If PostCSS finds source maps from a previous transformation, it will automatically update that source map with the same options.

If you want more control over source map generation, you can define the map option as an object with the following parameters:

  • inline boolean: indicates that the source map should be embedded in the output CSS as a Base64-encoded comment. By default, it is true. But if all previous maps are external, not inline, PostCSS will not embed the map even if you do not set this option.

    If you have an inline source map, the result.map property will be empty, as the source map will be contained within the text of result.css.

  • prev string, object or boolean: source map content from a previous processing step (for example, Sass compilation). PostCSS will try to read the previous source map automatically (based on comments within the source CSS), but you can use this option to identify it manually. If desired, you can omit the previous map with prev: false.

  • sourcesContent boolean: indicates that PostCSS should set the origin content (for example, Sass source) of the source map. By default, it is true. But if all previous maps do not contain sources content, PostCSS will also leave it out even if you do not set this option.

  • annotation boolean or string: indicates that PostCSS should add annotation comments to the CSS. By default, PostCSS will always add a comment with a path to the source map. PostCSS will not add annotations to CSS files that do not contain any comments.

    By default, PostCSS presumes that you want to save the source map as opts.to + '.map' and will use this path in the annotation comment. A different path can be set by providing a string value for annotation.

    If you have set inline: true, annotation cannot be disabled.

Safe Mode

If you pass the safe: true option to the process or parse methods, PostCSS will try to correct any syntax errors that it finds in the CSS.

postcss.parse('a {');                 // will throw "Unclosed block"
postcss.parse('a {', { safe: true }); // will return CSS root for a {}

This is useful for legacy code filled with hacks. Another use-case is interactive tools with live input — for example, the Autoprefixer demo.