Customizing PostCSS Config

Default Behavior

Next.js compiles CSS for its built-in CSS support using PostCSS.

Out of the box, with no configuration, Next.js compiles CSS with the following transformations:

1. Autoprefixer automatically adds vendor prefixes to CSS rules (back to IE11).
2. Cross-browser Flexbox bugs are corrected to behave like the spec.
3. New CSS features are automatically compiled for Internet Explorer 11 compatibility:
   • all Property
   • Break Properties
   • font-variant Property
   • Gap Properties
   • Media Query Ranges

By default, Custom Properties (CSS variables) are not compiled for IE11 support.

CSS variables are not compiled because it is not possible to safely do so. If you must use variables, consider using something like Sass variables which are compiled away by Sass.

Note: To support Grid Layout, you need to enable grid: "autoplace" for Autoprefixer. See "Customizing Plugins" below.

Customizing Target Browsers

Next.js allows you to configure the target browsers (for Autoprefixer and compiled css features) through Browserslist.

To customize browserslist, create a browserslist key in your package.json like so:

{
  "browserslist": [">0.3%", "not ie 11", "not dead", "not op_mini all"]
}

You can use the browserl.ist tool to visualize what browsers you are targeting.

CSS Modules

No configuration is needed to support CSS Modules. To enable CSS Modules for a file, rename the file to have the extension .module.css.

You can learn more about Next.js' CSS Module support here.

Customizing Plugins

Warning: When you define a custom PostCSS configuration file, Next.js completely disables the default behavior. Be sure to manually configure all the features you need compiled, including Autoprefixer. You also need to install any plugins included in your custom configuration manually, i.e. npm install postcss-flexbugs-fixes.

To customize the PostCSS configuration, create a postcss.config.json file in the root of your project.

This is the default configuration used by Next.js:

{
  "plugins": [
    "postcss-flexbugs-fixes",
    [
      "postcss-preset-env",
      {
        "autoprefixer": {
          "flexbox": "no-2009"
        },
        "stage": 3,
        "features": {
          "custom-properties": false
        }
      }
    ]
  ]
}

Note: Next.js also allows the file to be named .postcssrc.json, or, to be read from the postcss key in package.json.

It is also possible to configure PostCSS with a postcss.config.js file, which is useful when you want to conditionally include plugins based on environment:

module.exports = {
  plugins:
    process.env.NODE_ENV === 'production'
      ? [
          'postcss-flexbugs-fixes',
          [
            'postcss-preset-env',
            {
              autoprefixer: {
                flexbox: 'no-2009',
              },
              stage: 3,
              features: {
                'custom-properties': false,
              },
            },
          ],
        ]
      : [
          // No transformations in development
        ],
}

Note: Next.js also allows the file to be named .postcssrc.js.

Do not use require() to import the PostCSS Plugins. Plugins must be provided as strings.

Note: If your postcss.config.js needs to support other non-Next.js tools in the same project, you must use the interoperable object-based format instead:

module.exports = {
  plugins: {
    'postcss-flexbugs-fixes': {},
    'postcss-preset-env': {
      autoprefixer: {
        flexbox: 'no-2009',
      },
      stage: 3,
      features: {
        'custom-properties': false,
      },
    },
  },
}