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Extract & Inline Critical-path CSS in HTML pages
HTML JavaScript CSS

README.md

critical NPM version Build Status Build status Dependency Status

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

Install

$ npm install --save critical

Build plugins

Demo projects

Usage

Include:

var critical = require('critical');

Generate and inline critical-path CSS

critical.generateInline({
    // 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,

    // Target for final HTML output
    htmlTarget: 'index-critical.html',

    // Target for generated critical-path CSS (which we inline)
    styleTarget: 'styles/main.css',

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

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

    // Prefix for asset directory
    pathPrefix: '/MySubfolderDocrot',
});

Generate critical-path CSS

Basic usage:

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

Generate and minify critical-path CSS:

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

Generate and return output via a callback:

critical.generate({
    base: 'test/',
    src: 'index.html',
    width: 1300,
    height: 900
}, function (err, output) {
    // You now have critical-path CSS
    // Works with and without dest specified
});

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
    }]
});

Inline <style> / critical CSS from generation

Basic usage:

critical.inline({
    base: 'test/',
    src: 'index-critical.html',
    dest: 'inlined.html'
});

Minify and inline stylesheets:

critical.inline({
    base: 'test/',
    src: 'index-critical.html',
    dest: 'inlined-minified.html',
    minify: true
});

Inline and return output via a callback:

critical.inline({
    base: 'test/',
    src: 'index-critical.html'
}, function (err, output){
    // You now have HTML with inlined critical-path CSS
    // Works with and without dest specified
});

Options

Name Type Description
base string 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
src string Location of the HTML source to be operated against
dest string Location of where to save the output of an operation
width integer (Generation only) Width of the target viewport
height integer (Generation only) Height of the target viewport
dimensions array (Generation only) an array of objects containing height and width.
minify boolean Enable minification of CSS output
extract boolean 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
styleTarget string (generateInline only) Destination for critical-path styles
htmlTarget string (generateInline only) Destination for (critical-path CSS) style-inlined HTML
inlineImages boolean Inline images (default: false)
maxImageFileSize integer Sets a max file size (in bytes) for base64 inlined images
pathPrefix string (defaults to /) Path to prepend CSS assets with. You must make this path absolute if you are going to be using critical in multiple target files in disparate directory depths. (eg. targeting both /index.html and /admin/index.html would require this path to start with / or it wouldn't work.)

CLI

critical works well with standard input.

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

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

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

Gulp

var critical = require('critical').stream;

// Generate & Inline Critical-path CSS
gulp.task('critical', function () {
    return gulp.src('dist/*.html')
        .pipe(critical({base: 'dist/',css: ['dist/styles/components.css','dist/styles/main.css']}))
        .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 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 generate 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 from 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 maintain a criticalCSS node module, which similar to Penthouse will find and output the critical-path CSS for your pages.

Is Critical stable and suitable for production use?

Many of the current tools around critical-path CSS are in an experimental stage and are constantly striving to improve. The same could be said of Critical. It hasn't been extensively tested on a ton of sites and it's very possible something may well break. 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.

License

Apache-2.0 © Addy Osmani

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