WebP in CSS

PostCSS plugin and tiny JS script (175 bytes) to use WebP in CSS background.

This tool will make your images 25% smaller for Chrome, Firefox, and Edge. Safari will download the bigger JPEG/PNG image.

You add require('webp-in-css/polyfill') to your JS bundle and write CSS like:

.logo {
  width: 30px;
  height: 30px;
  background: url(/logo.png);
}

The script will set webp or no-webp class on <body> and PostCSS plugin will generate:

.logo {
  width: 30px;
  height: 30px;
  background: url(/logo.webp) no-repeat;
}
body.webp .logo {
  background-image: url(/logo.webp);
}
body.no-webp .logo, body.no-js .logo {
  background-image: url(/logo.png);
}

If you want to use addNoJs option, you need manually set no-js class on <body>. Polyfill will remove this class, if JS is enabled in the browser. Polyfill should be inserted in the <head>, without async or defer attributes, before css. addNoJs option is enabled by default.

Usage

Step 1: Install tool:

npm install --save-dev postcss webp-in-css

Step 2: convert all your JPEG/PNG images to WebP by Squoosh. Set checkbox on Lossless for PNG images and remove it for JPEG.

We recommend Reduce palette for most of the PNG images.

Save WebP images in the same places of JPEG/PNG images: img/bg.png → img/bg.webp.

Step 3: use <picture> to insert WebP images in HTML:

- <img src="/screenshot.jpg" alt="Screenshot">
+ <picture>
+   <source srcset="/screenshot.webp" type="image/webp">
+   <img src="/screenshot.jpg" alt="Screenshot">
+ </picture>

Step 4: add JS script to your client-side JS bundle:

+ import 'webp-in-css/polyfill'

Since JS script is very small (142 bytes), the best way for landings is to inline it to HTML:

  </head>
  <body>
+   <script><%= readFile('node_modules/webp-in-css/polyfill.js') %></script>

Note, that you need to put <script> inside <body>, not <head>.

Step 5: check do you use PostCSS already in your bundler. You can check postcss.config.js in the project root, "postcss" section in package.json or postcss in bundle config.

If you don’t have it already, add PostCSS to your bundle:

For webpack see postcss-loader docs.
For Parcel create postcss.config.js file. It already has PostCSS support.
For Gulp check gulp-postcss docs.

Step 6: Add webp-in-css/plugin to PostCSS plugins:

module.exports = {
  plugins: [
+   require('webp-in-css/plugin'),
    require('autoprefixer')
  ]
}

If you use CSS Modules in webpack add modules: true option:

module.exports = {
  plugins: [
-   require('webp-in-css/plugin'),
+   require('webp-in-css/plugin')({ modules: true }),
    require('autoprefixer')
  ]
}

We also recommend to put all images from CSS to preload content:

+   <link rel="preload" as="image" type="image/webp" href="/logo.webp">
    <script><%= readFile('node_modules/webp-in-css/polyfill.js') %></script>
  </head>

PostCSS Options

module.exports = {
  plugins: [
    require('webp-in-css/plugin')({ /* options */ }),
  ]
}

modules boolean: wrap classes to :global() to support CSS Modules. false by default.
webpClass string: class name for browser with WebP support.
noWebpClass string: class name for browser without WebP support.
addNoJs boolean: add no-js class to selector. true by default.
noJsClass string: class name for browser without JS support.
check function: should return boolean if we need to change declaration, default:
```
decl => /\.(jpe?g|png)(?!(\.webp|.*[&?]format=webp))/i.test(decl.value)
```
rename function: get a new file name from old name, like (oldName: string) => string, then url(./image.png) → url(./image.png.webp). Often you will need to change check option too.
```
check: decl => /\.jpg/.test(decl.value) && !decl.value.includes("as=webp"),
rename: url => url.replace(".jpg", ".jpg?as=webp")
```

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