Critters is a Webpack plugin that inlines your app's critical CSS and lazy-loads the rest.
critters-webpack-plugin 
It's a little different from other options, because it doesn't use a headless browser to render content. This tradeoff allows Critters to be very fast and lightweight. It also means Critters inlines all CSS rules used by your document, rather than only those needed for above-the-fold content. For alternatives, see Similar Libraries.
Critters' design makes it a good fit when inlining critical CSS for prerendered/SSR'd Single Page Applications. It was developed to be an excellent compliment to prerender-loader, combining to dramatically improve first paint time for most Single Page Applications.
Features
- Fast - no browser, few dependencies
- Integrates with html-webpack-plugin
- Works with
webpack-dev-server/webpack serve - Supports preloading and/or inlining critical fonts
- Prunes unused CSS keyframes and media queries
- Removes inlined CSS rules from lazy-loaded stylesheets
Installation
First, install Critters as a development dependency:
npm i -D critters-webpack-plugin
Then, import Critters into your Webpack configuration and add it to your list of plugins:
// webpack.config.js
+const Critters = require('critters-webpack-plugin');
module.exports = {
plugins: [
+ new Critters({
+ // optional configuration (see below)
+ })
]
}
That's it! Now when you run Webpack, the CSS used by your HTML will be inlined and the imports for your full CSS will be converted to load asynchronously.
Usage
Critters
Create a Critters plugin instance with the given options.
Parameters
optionsOptions Options to control how Critters inlines CSS.
Examples
// webpack.config.js
module.exports = {
plugins: [
new Critters({
// Outputs: <link rel="preload" onload="this.rel='stylesheet'">
preload: 'swap',
// Don't inline critical font-face rules, but preload the font URLs:
preloadFonts: true
})
]
}
Critters
All optional. Pass them to new Critters({ ... }).
Parameters
options
Properties
externalBoolean Inline styles from external stylesheets (default:true)inlineThresholdNumber Inline external stylesheets smaller than a given size (default:0)minimumExternalSizeNumber If the non-critical external stylesheet would be below this size, just inline it (default:0)pruneSourceBoolean Remove inlined rules from the external stylesheet (default:true)mergeStylesheetsBoolean Merged inlined stylesheets into a
single tag (default:true)additionalStylesheetsString[] Glob for matching other stylesheets which should be used to evaluate critical CSS (default: '')preloadString Which preload strategy to usenoscriptFallbackBoolean Add<noscript>fallback to JS-based strategiesinlineFontsBoolean Inline critical font-face rules (default:false)preloadFontsBoolean Preloads critical fonts (default:true)fontsBoolean Shorthand for settinginlineFonts+preloadFonts- Values:trueto inline critical font-face rules and preload the fontsfalseto don't inline any font-face rules and don't preload fonts
keyframesString Controls which keyframes rules are inlined.- Values:"critical": (default) inline keyframes rules used by the critical CSS"all"inline all keyframes rules"none"remove all keyframes rules
compressBoolean Compress resulting critical CSS (default:true)logLevelString Controls log level of the plugin (default:"info")
LogLevel
Controls log level of the plugin. Specifies the level the logger should use. A logger will
not produce output for any log level beneath the specified level. Available levels and order
are:
- "info" (default)
- "warn"
- "error"
- "trace"
- "debug"
- "silent"
Type: ("info" , "warn" , "error" , "trace" , "debug" , "silent")
PreloadStrategy
The mechanism to use for lazy-loading stylesheets.
[JS] indicates that a strategy requires JavaScript (falls back to <noscript>).
- default: Move stylesheet links to the end of the document and insert preload meta tags in their place.
- "body": Move all external stylesheet links to the end of the document.
- "media": Load stylesheets asynchronously by adding
media="not x"and removing once loaded. [JS] - "swap": Convert stylesheet links to preloads that swap to
rel="stylesheet"once loaded. [JS] - "js": Inject an asynchronous CSS loader similar to LoadCSS and use it to load stylesheets. [JS]
- "js-lazy": Like
"js", but the stylesheet is disabled until fully loaded.
Type: (default, "body" , "media" , "swap" , "js" , "js-lazy")
Similar Libraries
There are a number of other libraries that can inline Critical CSS, each with a slightly different approach. Here are a few great options:
License
This is not an official Google product.