Critters

Critters is a plugin that inlines your app's critical CSS and lazy-loads the rest.

critters

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 Webpack critters-webpack-plugin
• 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

or

yarn add -D critters

Simple Example

import Critters from 'critters';

const critters = new Critters({
  // optional configuration (see below)
});

const html = `
  <style>
    .red { color: red }
    .blue { color: blue }
  </style>
  <div class="blue">I'm Blue</div>
`;

const inlined = await critters.process(html);

console.log(inlined);
// "<style>.blue{color:blue}</style><div class=\"blue\">I'm Blue</div>"

Usage with webpack

Critters is also available as a Webpack plugin called critters-webpack-plugin.

The Webpack plugin supports the same configuration options as the main critters package:

// webpack.config.js
+const Critters = require('critters-webpack-plugin');

module.exports = {
  plugins: [
+    new Critters({
+      // optional configuration (see below)
+    })
  ]
}

That's it! The resultant html will have its critical CSS inlined and the stylesheets lazy-loaded.

Usage

Critters

All optional. Pass them to new Critters({ ... }).

Parameters

• options

Properties

• path String Base path location of the CSS files (default: '')
• publicPath String Public path of the CSS resources. This prefix is removed from the href (default: '')
• external Boolean Inline styles from external stylesheets (default: true)
• inlineThreshold Number Inline external stylesheets smaller than a given size (default: 0)
• minimumExternalSize Number If the non-critical external stylesheet would be below this size, just inline it (default: 0)
• pruneSource Boolean Remove inlined rules from the external stylesheet (default: false)
• mergeStylesheets Boolean Merged inlined stylesheets into a single <style> tag (default: true)
• additionalStylesheets Array<String> Glob for matching other stylesheets to be used while looking for critical CSS.
• preload String Which preload strategy to use
• noscriptFallback Boolean Add <noscript> fallback to JS-based strategies
• inlineFonts Boolean Inline critical font-face rules (default: false)
• preloadFonts Boolean Preloads critical fonts (default: true)
• fonts Boolean Shorthand for setting inlineFonts + preloadFonts* Values:
  • true to inline critical font-face rules and preload the fonts
  • false to don't inline any font-face rules and don't preload fonts
• keyframes String 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
• compress Boolean Compress resulting critical CSS (default: true)
• logLevel String Controls log level of the plugin (default: "info")
• logger object Provide a custom logger interface logger

Logger

Custom logger interface:

Type: object

Properties

• trace function (String) Prints a trace message
• debug function (String) Prints a debug message
• info function (String) Prints an information message
• warn function (String) Prints a warning message
• error function (String) Prints an error message

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.

Note: JS indicates a strategy requiring JavaScript (falls back to <noscript> unless disabled).

• 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 (details). JS
• "swap-high": Use <link rel="alternate stylesheet preload"> and swap to rel="stylesheet" once loaded (details). 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.
• false: Disables adding preload tags.

Type: (default | "body" | "media" | "swap" | "swap-high" | "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:

• Critical
• Penthouse
• webpack-critical
• webpack-plugin-critical
• html-critical-webpack-plugin
• react-snap

License

Apache 2.0

This is not an official Google product.