brschwar / critical

Extract & Inline Critical-path CSS from HTML (alpha)

critical

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

Install

$ npm install --save critical

Demo projects

• Optimize a basic page with Gulp with a tutorial
• Optimize an Angular boilerplate with Gulp
• Optimize a Weather app with Gulp

Usage

Include:

var critical = require('critical');

Generate and inline critical-path CSS

critical.generateInline({
    // Your base directory
    base: 'dist/',
    
    // HTML source
    src: 'index.html',

    // Your CSS Files (optional)
    css: ['dist/styles/main.css'],
    
    // Viewport width
    width: 320,
    
    // Viewport height
    height: 480,
    
    // 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
});

Generate critical-path CSS

Basic usage:

critical.generate({
    base: 'test/',
    src: 'index.html',
    dest: 'styles/main.css',
    width: 320,
    height: 480
});

Generate and minify critical-path CSS:

critical.generate({
    base: 'test/',
    src: 'index.html',
    dest: 'styles/styles.min.css',
    minify: true,
    width: 320,
    height: 480
});

Generate and return output via a callback:

critical.generate({
    base: 'test/',
    src: 'index.html',
    width: 320,
    height: 480
}, function (err, output) {
    // You now have critical-path CSS
    // Works with and without dest specified
});

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
src	string	Location of the HTML source to be operated against
dest	string	Location of where to save the output of an operation
css	array	Use this CSS files instead of the ones exctracted from the HTML
width	integer	(Generation only) Width of the target viewport
height	integer	(Generation only) Height of the target viewport
minify	boolean	Enable minification of CSS output
styleTarget	string	(generateInline only) Destination for critical-path styles
htmlTarget	string	(generateInline only) Destination for (critical-path CSS) style-inlined HTML

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.