lightningcss-wasm

⚡️ Lightning CSS

An extremely fast CSS parser, transformer, and minifier written in Rust. Use it with Parcel, as a standalone library or CLI, or via a plugin with any other tool.

Features

• Extremely fast – Parsing and minifying large files is completed in milliseconds, often with significantly smaller output than other tools. See benchmarks below.
• Typed property values – many other CSS parsers treat property values as an untyped series of tokens. This means that each transformer that wants to do something with these values must interpret them itself, leading to duplicate work and inconsistencies. Lightning CSS parses all values using the grammar from the CSS specification, and exposes a specific value type for each property.
• Browser-grade parser – Lightning CSS is built on the cssparser and selectors crates created by Mozilla and used by Firefox and Servo. These provide a solid general purpose CSS-parsing foundation on top of which Lightning CSS implements support for all specific CSS rules and properties.
• Minification – One of the main purposes of Lightning CSS is to minify CSS to make it smaller. This includes many optimizations including:
  • Combining longhand properties into shorthands where possible.
  • Merging adjacent rules with the same selectors or declarations when it is safe to do so.
  • Combining CSS transforms into a single matrix or vice versa when smaller.
  • Removing vendor prefixes that are not needed, based on the provided browser targets.
  • Reducing calc() expressions where possible.
  • Converting colors to shorter hex notation where possible.
  • Minifying gradients.
  • Minifying CSS grid templates.
  • Normalizing property value order.
  • Removing default property sub-values which will be inferred by browsers.
  • Many micro-optimizations, e.g. converting to shorter units, removing unnecessary quotation marks, etc.
• Vendor prefixing – Lightning CSS accepts a list of browser targets, and automatically adds (and removes) vendor prefixes.
• Syntax lowering – Lightning CSS parses modern CSS syntax, and generates more compatible output where needed, based on browser targets.
  • CSS Nesting (draft spec)
  • Custom media queries (draft spec)
  • Logical properties
  • Color Level 5
  • color-mix() function
  • Color Level 4
    • lab(), lch(), oklab(), and oklch() colors
    • color() function supporting predefined color spaces such as display-p3 and xyz
    • Space separated components in rgb and hsl functions
    • Hex with alpha syntax
    • hwb() color syntax
    • Percent syntax for opacity
    • #rgba and #rrggbbaa hex colors
  • Selectors
    • :not with multiple arguments
    • :lang with multiple arguments
    • :dir
    • :is
  • Double position gradient stops (e.g. red 40% 80%)
  • clamp(), round(), rem(), and mod() math functions
  • Alignment shorthands (e.g. place-items)
  • Two-value overflow shorthand
  • Media query range syntax (e.g. @media (width <= 100px) or @media (100px < width < 500px))
  • Multi-value display property (e.g. inline flex)
  • system-ui font family fallbacks
• CSS modules – Lightning CSS supports compiling a subset of CSS modules features.
  • Locally scoped class and id selectors
  • Locally scoped custom identifiers, e.g. @keyframes names, grid lines/areas, @counter-style names, etc.
  • Opt-in support for locally scoped CSS variables and other dashed identifiers.
  • :local() and :global() selectors
  • The composes property

Documentation

Lightning CSS can be used from Parcel, as a standalone library from JavaScript or Rust, using a standalone CLI, or wrapped as a plugin within any other tool.

From Node

See the TypeScript definitions for full API docs.

Here is a simple example that compiles the input CSS for Safari 13.2, and minifies the output.

const css = require('lightningcss');

let {code, map} = css.transform({
  filename: 'style.css',
  code: Buffer.from('.foo { color: red }'),
  minify: true,
  sourceMap: true,
  targets: {
    // Semver versions are represented using a single 24-bit number, with one component per byte.
    // e.g. to represent 13.2.0, the following could be used.
    safari: (13 << 16) | (2 << 8)
  }
});

You can also convert the results of running browserslist into targets which can be passed to Lightning CSS:

const browserslist = require('browserslist');
const css = require('lightningcss');

let targets = css.browserslistToTargets(browserslist('>= 0.25%'));

Bundling is also possible by using the bundle API. This processes @import rules and inlines them. This API requires filesystem access, so it does not accept code directly via the API.

let {code, map} = css.bundle({
  filename: 'style.css',
  minify: true
});

The bundleAsync API is an asynchronous version of bundle, which also accepts a custom resolver object. This allows you to provide custom JavaScript functions for resolving @import specifiers to file paths, and reading files from the file system (or another source). The read and resolve functions are both optional, and may either return a string synchronously, or a Promise for asynchronous resolution.

let {code, map} = await css.bundleAsync({
  filename: 'style.css',
  minify: true,
  resolver: {
    read(filePath) {
      return fs.readFileSync(filePath, 'utf8');
    },
    resolve(specifier, from) {
      return path.resolve(path.dirname(from), specifier);
    }
  }
});

Note that using a custom resolver can slow down bundling significantly, especially when reading files asynchronously. Use readFileSync rather than readFile if possible for better performance, or omit either of the methods if you don't need to override the default behavior.

From Rust

See the Rust API docs on docs.rs.

With Parcel

Parcel includes Lightning CSS as the default CSS transformer. You should also add a browserslist property to your package.json, which defines the target browsers that your CSS will be compiled for.

While Lightning CSS handles the most commonly used PostCSS plugins like autoprefixer, postcss-preset-env, and CSS modules, you may still need PostCSS for more custom plugins like TailwindCSS. If that's the case, your PostCSS config will be picked up automatically. You can remove the plugins listed above from your PostCSS config, and they'll be handled by Lightning CSS.

You can also configure Lightning CSS in the package.json in the root of your project. Currently, three options are supported: drafts, which can be used to enable CSS nesting and custom media queries, pseudoClasses, which allows replacing some pseudo classes like :focus-visible with normal classes that can be applied via JavaScript (e.g. polyfills), and cssModules, which enables CSS modules globally rather than only for files ending in .module.css, or accepts an options object.

{
  "@parcel/transformer-css": {
    "cssModules": true,
    "drafts": {
      "nesting": true,
      "customMedia": true
    },
    "pseudoClasses": {
      "focusVisible": "focus-ring"
    }
  }
}

See the Parcel docs for more details.

From Deno or in browser

The lightningcss-wasm package can be used in Deno or directly in browsers. This uses a WebAssembly build of Lightning CSS. Use TextEncoder and TextDecoder convert code from a string to a typed array and back.

import init, {transform} from 'https://unpkg.com/lightningcss-wasm';

await init();

let {code, map} = transform({
  filename: 'style.css',
  code: new TextEncoder().encode('.foo { color: red }'),
  minify: true,
});

console.log(new TextDecoder().decode(code));

With webpack

css-minimizer-webpack-plugin has builtin support for Lightning CSS. Install Lightning CSS in your project, and configure the plugin as documented here.

From the CLI

Lightning CSS includes a standalone CLI that can be used to compile, minify, and bundle CSS files. It can be used when you only need to compile CSS, and don't need more advanced functionality from a larger build tool such as code splitting and support for other languages.

To use the CLI, install the lightningcss-cli package with an npm compatible package manager:

npm install lightningcss-cli

Then, you can run the lightningcss command via npx, yarn, or by setting up a script in your package.json.

{
  "scripts": {
    "build": "lightningcss --minify --nesting --bundle --targets '>= 0.25%' --sourcemap input.css -o output.css"
  }
}

To see all of the available options, use the --help argument:

npx lightningcss --help

Error recovery

By default, Lightning CSS is strict, and will error when parsing an invalid rule or declaration. However, sometimes you may encounter a third party library that you can't easily modify, which unintentionally contains invalid syntax, or IE-specific hacks. In these cases, you can enable the errorRecovery option (or --error-recovery CLI flag). This will skip over invalid rules and declarations, omitting them in the output, and producing a warning instead of an error. You should also open an issue or PR to fix the issue in the library if possible.

Benchmarks

$ node bench.js bootstrap-4.css 
cssnano: 544.809ms
159636 bytes

esbuild: 17.199ms
160332 bytes

lightningcss: 4.16ms
143091 bytes


$ node bench.js animate.css
cssnano: 283.105ms
71723 bytes

esbuild: 11.858ms
72183 bytes

lightningcss: 1.973ms
23666 bytes


$ node bench.js tailwind.css 
cssnano: 2.198s
1925626 bytes

esbuild: 107.668ms
1961642 bytes

lightningcss: 43.368ms
1824130 bytes

For more benchmarks comparing more tools and input, see here. Note that some of the tools shown perform unsafe optimizations that may change the behavior of the original CSS in favor of smaller file size. Lightning CSS does not do this – the output CSS should always behave identically to the input. Keep this in mind when comparing file sizes between tools.