PostCSS image-set() Function

PostCSS image-set() Function lets you display resolution-dependent images using the image-set() function in CSS, following the CSS Images specification.

.example {
  background-image: image-set(
    url(img.png) 1x,
    url(img@2x.png) 2x,
    url(img@print.png) 600dpi
  );
}

/* becomes */

.example {
  background-image: url(img.png);
}

@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  .example {
    background-image: url(img@2x.png);
  }
}


@media (-webkit-min-device-pixel-ratio: 6.25), (min-resolution: 600dpi) {
  .example {
    background-image: url(my@print.png);
  }
}

.example {
  background-image: image-set(
    url(img.png) 1x,
    url(img@2x.png) 2x,
    url(img@print.png) 600dpi
  );
}

Add PostCSS image-set() Function to your build tool:

npm install postcss-image-set-function --save-dev

Use PostCSS image-set() Function to process your CSS:

import postcssImageSetFunction from 'postcss-image-set-function';

postcssImageSetFunction.process(YOUR_CSS, /* processOptions */, /* pluginOptions */);

Add PostCSS to your build tool:

npm install postcss --save-dev

Use PostCSS image-set() Function as a plugin:

import postcss from 'gulp-postcss';
import postcssImageSetFunction from 'postcss-image-set-function';

postcss([
  postcssImageSetFunction(/* pluginOptions */)
]).process(YOUR_CSS);

Add PostCSS Loader to your build tool:

npm install postcss-loader --save-dev

Use PostCSS image-set() Function in your Webpack configuration:

import postcssImageSetFunction from 'postcss-image-set-function';

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          'style-loader',
          { loader: 'css-loader', options: { importLoaders: 1 } },
          { loader: 'postcss-loader', options: {
            ident: 'postcss',
            plugins: () => [
              postcssImageSetFunction(/* pluginOptions */)
            ]
          } }
        ]
      }
    ]
  }
}

Add Gulp PostCSS to your build tool:

npm install gulp-postcss --save-dev

Use PostCSS image-set() Function in your Gulpfile:

import postcss from 'gulp-postcss';
import postcssImageSetFunction from 'postcss-image-set-function';

gulp.task('css', () => gulp.src('./src/*.css').pipe(
  postcss([
    postcssImageSetFunction(/* pluginOptions */)
  ])
).pipe(
  gulp.dest('.')
));

Add Grunt PostCSS to your build tool:

npm install grunt-postcss --save-dev

Use PostCSS image-set() Function in your Gruntfile:

import postcssImageSetFunction from 'postcss-image-set-function';

grunt.loadNpmTasks('grunt-postcss');

grunt.initConfig({
  postcss: {
    options: {
      use: [
       postcssImageSetFunction(/* pluginOptions */)
      ]
    },
    dist: {
      src: '*.css'
    }
  }
});

The preserve option determines whether the original declaration using image-set() is preserved. By default, it is preserved.

postcssImageSetFunction({ preserve: false })

.example {
  background-image: image-set(
    url(img.png) 1x,
    url(img@2x.png) 2x,
    url(img@print.png) 600dpi
  );
}

/* becomes */

.example {
  background-image: url(img.png);
}

@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  .example {
    background-image: url(img@2x.png);
  }
}


@media (-webkit-min-device-pixel-ratio: 6.25), (min-resolution: 600dpi) {
  .example {
    background-image: url(my@print.png);
  }
}

The oninvalid option determines how invalid usage of image-set() should be handled. By default, invalid usages of image-set() are ignored. They can be configured to display a warning or throw an error.

postcssImageSetFunction({ oninvalid: 'warning' }) // warn on invalid usages

postcssImageSetFunction({ oninvalid: 'throw' }) // throw on invalid usages

The image-set() function allows an author to provide multiple resolutions of an image and let the browser decide which is most appropriate in a given situation. The image-set() also never fails to choose an image; the <resolution> just helps determine which of the images is chosen.

Since this plugin is not a browser, the image options are sorted by device pixel ratio and the lowest ratio is used as the default, while the remaining images are pushed behind media queries.

Therefore, this plugin can only approximate native browser behavior. While images should typically match the resolution as the device they’re being viewed in, other factors can affect the chosen image. For example, if the user is on a slow mobile connection, the browser may prefer to select a lower-res image rather than wait for a larger, resolution-matching image to load.