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Builtin lightningcss-loader

website/docs/en/guide/features/builtin-lightningcss-loader.mdx

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import { ApiMeta, Stability } from '@components/ApiMeta';

Builtin lightningcss-loader

<ApiMeta addedVersion="1.0.0" />

Lightning CSS is a high performance CSS parser, transformer and minifier written in Rust. It supports parsing and transforming many modern CSS features into syntax supported by target browsers, and also provides a better compression ratio.

Rspack provides a built-in builtin:lightningcss-loader, which is based on Lightning CSS to transform CSS. It can replace the postcss-loader and autoprefixer for CSS syntax downgrading, prefixing, and other functionalities, offering better performance.

::: warning Please note that Lightning CSS strictly requires standards-compliant CSS input. When non-standard CSS is processed by the builtin:lightningcss-loader, styles may be ignored or produce unexpected results (Undefined Behavior). To ensure that styles are correctly applied, avoid using non-standard CSS syntax or browser-specific proprietary syntax, and instead use standard CSS writing practices that conform to W3C specifications. :::

Example

To use builtin:lightningcss-loader in your project, you need to configure it as follows.

js
import { rspack } from '@rspack/core';

export default {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          {
            loader: 'builtin:lightningcss-loader',
            options: {
              targets: 'ie 10',
            },
          },
          // ... other loaders
        ],
      },
    ],
  },
};

Type declarations

You can use the LightningcssLoaderOptions type exported by @rspack/core to enable type hints:

js
export default {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          {
            loader: 'builtin:lightningcss-loader',
            /** @type {import('@rspack/core').LightningcssLoaderOptions} */
            options: {
              targets: 'ie 10',
            },
          },
          // ... other loaders
        ],
      },
    ],
  },
};

Options

Below are the configurations supported by builtin:lightningcss-loader. For detailed configuration, please refer to lightningcss document.

ts
type LightningcssFeatureOptions = {
  nesting?: boolean;
  notSelectorList?: boolean;
  dirSelector?: boolean;
  langSelectorList?: boolean;
  isSelector?: boolean;
  textDecorationThicknessPercent?: boolean;
  mediaIntervalSyntax?: boolean;
  mediaRangeSyntax?: boolean;
  customMediaQueries?: boolean;
  clampFunction?: boolean;
  colorFunction?: boolean;
  oklabColors?: boolean;
  labColors?: boolean;
  p3Colors?: boolean;
  hexAlphaColors?: boolean;
  spaceSeparatedColorNotation?: boolean;
  fontFamilySystemUi?: boolean;
  doublePositionGradients?: boolean;
  vendorPrefixes?: boolean;
  logicalProperties?: boolean;
  selectors?: boolean;
  mediaQueries?: boolean;
  color?: boolean;
};

type Targets = {
  android?: string;
  chrome?: string;
  edge?: string;
  firefox?: string;
  ie?: string;
  ios_saf?: string;
  opera?: string;
  safari?: string;
  samsung?: string;
};

type LightningcssLoaderOptions = {
  minify?: boolean;
  errorRecovery?: boolean;
  targets?: string[] | string | Targets;
  include?: LightningcssFeatureOptions;
  exclude?: LightningcssFeatureOptions;
  drafts?: Drafts;
  nonStandard?: NonStandard;
  pseudoClasses?: PseudoClasses;
  unusedSymbols?: Set<String>;
};

targets

  • Type: string | string[] | Targets

Browserslist query string or a Targets object specifying browser versions.

:::tip Default targets from Rspack If targets is not configured, builtin:lightningcss-loader will automatically derive a default targets from Rspack's target configuration when using browserslist-related targets (e.g., browserslist or browserslist:modern). Since Lightning CSS only supports browser-related targets, non-browser targets like node will not provide default targets for this loader.

This means you can rely on your Rspack target configuration without manually specifying the same targets in the loader options. :::

Here are some examples of setting targets.

  • Setting a browserslist query string:
js
const loader = {
  loader: 'builtin:lightningcss-loader',
  /** @type {import('@rspack/core').LightningcssLoaderOptions} */
  options: {
    targets: 'ie 10',
  },
};
  • Setting an array of browserslist query strings:
js
const loader = {
  loader: 'builtin:lightningcss-loader',
  /** @type {import('@rspack/core').LightningcssLoaderOptions} */
  options: {
    targets: ['chrome >= 87', 'edge >= 88', '> 0.5%'],
  },
};
  • Setting a Targets object:
js
const loader = {
  loader: 'builtin:lightningcss-loader',
  /** @type {import('@rspack/core').LightningcssLoaderOptions} */
  options: {
    targets: {
      chrome: '95.0',
      safari: '13.2',
    },
  },
};

errorRecovery

  • Type: boolean
  • Default: true

Control how Lightning CSS handles invalid CSS syntax.

By default, this option is enabled, meaning that when invalid CSS rules or declarations are parsed, Lightning CSS will skip them and emit warnings, while omitting them from the final output without interrupting the compilation process.

Ignoring warnings

If you confirm that these warnings can be ignored, you can use ignoreWarnings to filter out the warnings from LightningCSS.

For example, ignore all warnings:

js
export default {
  ignoreWarnings: [
    (warning) => /LightningCSS parse warning/.test(warning.message),
  ],
};

You can also use regular expressions to ignore specific warnings.

Disabling errorRecovery

If you set errorRecovery to false, Lightning CSS will throw a compilation error and interrupt the build process when parsing any invalid CSS syntax:

js
export default {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          {
            loader: 'builtin:lightningcss-loader',
            /** @type {import('@rspack/core').LightningcssLoaderOptions} */
            options: {
              errorRecovery: false,
            },
          },
        ],
      },
    ],
  },
};