Critical CSS Plugin: Extract & Inline Critical-path CSS In HTML Pages

Critical plugin extracts & inlines critical-path (above-the-fold) CSS from HTML.

Tabel of Contents

Install
Breaking Changes
Build Plugins
Demo Projects
Usage
CLI
Gulp
Why?
FAQ
Can I contribute?
Maintainers
License

Install

Install current beta version

npm i -D critical@next

Install latest stable version

npm i -D critical@latest

The docs for the latest 1.x version can be found here.

Breaking Changes

We’ve introduced some breaking changes in this release so be sure to check out the changelog.

Build plugins

grunt-critical
Gulp users should use Critical directly
For Webpack use html-critical-webpack-plugin

Demo projects

Usage

Include:

const critical = require('critical');

Full blown example with available options:

critical.generate({
  // Inline the generated critical-path CSS
  // - true generates HTML
  // - false generates CSS
  inline: true,

  // Your base directory
  base: 'dist/',

  // HTML source
  html: '<html>...</html>',

  // HTML source file
  src: 'index.html',

  // Your CSS Files (optional)
  css: ['dist/styles/main.css'],

  // Viewport width
  width: 1300,

  // Viewport height
  height: 900,

  // Output results to file
  target: {
    css: 'critical.css',
    html: 'index-critical.html',
    uncritical: 'uncritical.css'
  },

  // Minify critical-path CSS when inlining
  minify: true,

  // Extract inlined styles from referenced stylesheets
  extract: true,

  // Complete Timeout for Operation
  timeout: 30000,

  // ignore CSS rules
  ignore: {
    atrule: ['@font-face'],
    rule: [/some-regexp/],
    decl: (node, value) => /big-image\.png/.test(value)
  }
});

Generate and inline critical-path CSS

Basic usage:

critical.generate({
  inline: true,
  base: 'test/',
  src: 'index.html',
  target: 'index-critical.html',
  width: 1300,
  height: 900
});

Generate critical-path CSS

Basic usage:

critical.generate({
  base: 'test/',
  src: 'index.html',
  target: 'styles/main.css',
  width: 1300,
  height: 900
});

Generate and minify critical-path CSS:

critical.generate({
  base: 'test/',
  src: 'index.html',
  target: 'styles/styles.min.css',
  width: 1300,
  height: 900
});

Generate, minify and inline critical-path CSS:

critical.generate({
    inline: true,
    base: 'test/',
    src: 'index.html',
    target: {
      html: 'index-critical.html',
      css: 'critical.css'
    },
    width: 1300,
    height: 900
});

Generate and return output via callback:

critical.generate({
    base: 'test/',
    src: 'index.html',
    width: 1300,
    height: 900,
    inline: true
}, (err, ({css, html, uncritical})) => {
    // You now have critical-path CSS as well as the modified HTML.
    // Works with and without target specified.
    ...
});

Generate and return output via promise:

critical.generate({
    base: 'test/',
    src: 'index.html',
    width: 1300,
    height: 900
}).then((({css, html, uncritical})) => {
    // You now have critical-path CSS as well as the modified HTML.
    // Works with and without target specified.
    ...
}).error(err => {
    ...
});

Generate and return output via async function:

const {css, html, uncritical} = await critical.generate({
  base: 'test/',
  src: 'index.html',
  width: 1300,
  height: 900
});

Generate critical-path CSS with multiple resolutions

When your site is adaptive and you want to deliver critical CSS for multiple screen resolutions this is a useful option. note: (your final output will be minified as to eliminate duplicate rule inclusion)

critical.generate({
  base: 'test/',
  src: 'index.html',
  dest: 'styles/main.css',
  dimensions: [{
      height: 200,
      width: 500
    },
    {
      height: 900,
      width: 1200
  }]
});

Generate critical-path CSS and ignore specific selectors

This is a useful option when you e.g. want to defer loading of webfonts or background images.

critical.generate({
  base: 'test/',
  src: 'index.html',
  dest: 'styles/main.css',
  ignore: {
    atrule: ['@font-face'],
    decl: (node, value) => /url\(/.test(value)
  }
});

Generate critical-path CSS and specify asset rebase behaviour

critical.generate({
  base: 'test/',
  src: 'index.html',
  dest: 'styles/main.css',
  rebase: {
    from: '/styles/main.css',
    to: '/folder/subfolder/index.html'
  }
});

critical.generate({
  base: 'test/',
  src: 'index.html',
  dest: 'styles/main.css',
  rebase: asset => `https://my-cdn.com${asset.absolutePath}`
});

Options

Name	Type	Default	Description
inline	`boolean`\|`object`	`false`	Inline critical-path CSS using filamentgroup’s loadCSS. Pass an object to configure `inline-critical`
base	`string`	`path.dirname(src)` or `process.cwd()`	Base directory in which the source and destination are to be written
html	`string`		HTML source to be operated against. This option takes precedence over the `src` option.
css	`array`	`[]`	An array of paths to css files, file globs or Vinyl file objects.
src	`string`		Location of the HTML source to be operated against
target	`string` or `object`		Location of where to save the output of an operation. Use an object with ‘html’ and ‘css’ props if you want to store both
width	`integer`	`1300`	Width of the target viewport
height	`integer`	`900`	Height of the target viewport
dimensions	`array`	`[]`	An array of objects containing height and width. Takes precedence over `width` and `height` if set
minify	`boolean`	`true`	Enable minification of generated critical-path CSS
extract	`boolean`	`false`	Remove the inlined styles from any stylesheets referenced in the HTML. It generates new references based on extracted content so it’s safe to use for multiple HTML files referencing the same stylesheet. Use with caution. Removing the critical CSS per page results in a unique async loaded CSS file for every page. Meaning you can’t rely on cache across multiple pages
inlineImages	`boolean`	`false`	Inline images
assetPaths	`array`	`[]`	List of directories/urls where the inliner should start looking for assets
maxImageFileSize	`integer`	`10240`	Sets a max file size (in bytes) for base64 inlined images
rebase	`object` or `function`	`undefined`	Critical tries it’s best to rebase the asset paths relative to the document. If this doesn’t work as expected you can always use this option to control the rebase paths. See `postcss-url` for details. (https://github.com/pocketjoso/penthouse#usage-1).
ignore	`array`	`object`	`undefined`
userAgent	`string`	`''`	User agent to use when fetching a remote src
penthouse	`object`	`{}`	Configuration options for `penthouse`.
user	`string`	`undefined`	RFC2617 basic authorization: user
pass	`string`	`undefined`	RFC2617 basic authorization: pass
strict	`boolean`	`false`	Throw an error if no css is found

CLI

npm install -g critical

critical works well with standard input.

cat test/fixture/index.html | critical --base test/fixture --inline > index.critical.html

Or on Windows:

type test\fixture\index.html | critical --base test/fixture --inline > index.critical.html

You can also pass in the critical CSS file as an option.

critical test/fixture/index.html --base test/fixture > critical.css

Gulp

const gulp = require('gulp');
const log = require('fancy-log');
const critical = require('critical').stream;

// Generate & Inline Critical-path CSS
gulp.task('critical', () => {
  return gulp
    .src('dist/*.html')
    .pipe(critical({
      base: 'dist/',
      inline: true,
      css: [
        'dist/styles/components.css',
        'dist/styles/main.css'
      ]
    }))
    .on('error', err => {
      log.error(err.message);
    })
    .pipe(gulp.dest('dist'));
});

Why?

Why is critical-path CSS important?

CSS is required to construct the render tree for your pages and JavaScript will often block on CSS during the initial construction of the page. You should ensure that any non-essential CSS is marked as non-critical (e.g. print and other media queries) and that the amount of critical CSS and the time to deliver it is as small as possible.

Why should critical-path CSS be inlined?

For best performance, you may want to consider inlining the critical CSS directly into the HTML document. This eliminates additional roundtrips in the critical path and if done correctly can be used to deliver a “one roundtrip” critical path length where only the HTML is a blocking resource.

FAQ

Are there any sample projects available using Critical?

Why, yes!. Take a look at this Gulp project which demonstrates using Critical to generating and inline critical-path CSS. It also includes a mini-tutorial that walks through how to use it in a simple webapp.

When should I just use Penthouse directly?

The main differences between Critical and Penthouse, a module we use, are:

Critical will automatically extract stylesheets from your HTML which to generate critical-path CSS from, whilst other modules generally require you to specify this upfront.
Critical provides methods for inlining critical-path CSS (a common logical next-step once your CSS is generated)
Since we tackle both generation and inlining, we’re able to abstract away some of the ugly boilerplate otherwise involved in tackling these problems separately.

That said, if your site or app has a large number of styles or styles which are being dynamically injected into the DOM (sometimes common in Angular apps) I recommend using Penthouse directly. It will require you to supply styles upfront, but this may provide a higher level of accuracy if you find Critical isn’t serving your needs.

What other alternatives to Critical are available?

FilamentGroup maintains a criticalCSS node module, which similar to Penthouse will find and output the critical-path CSS for your pages. The PageSpeed Optimization modules for Nginx, Apache, IIS, ATS, and Open Lightspeed can do all the heavy lifting automatically when you enable the prioritize_critical_css filter

Is Critical stable and suitable for production use?

Critical has been used on a number of production sites that have found it stable for everyday use. That said, we welcome you to try it out on your project and report bugs if you find them.

Can I contribute?

Of course. We appreciate all of our contributors and welcome contributions to improve the project further. If you’re uncertain whether an addition should be made, feel free to open up an issue and we can discuss it.

Maintainers

This module is brought to you and maintained by the following people:

Addy Osmani – Creator (Github / Twitter)
Ben Zörb – Primary maintainer (Github / Twitter)

License

Icons made by Flat Icons from www.flaticon.com

Critical CSS plugin: Extract & Inline Critical-path CSS in HTML pages