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Filament Group

Finds the Above the Fold CSS for your page, and outputs it into a file

Getting Started

Install the module with: npm install criticalcss

var criticalcss = require("criticalcss");

criticalcss.findCritical("path/to/file/or/url", options (only rules required), function(err, output){
	if( err ){
		throw new Error( err );
	} else {
		fs.writeFileSync( filename, output );

criticalcss.getRules("path/to/css/file/", function(err, output){
	if( err ){
		throw new Error( err );
	} else {
		fs.writeFileSync( filename, output );

Using both together:

var request = require('request');
var path = require( 'path' );
var criticalcss = require("criticalcss");
var fs = require('fs');
var tmpDir = require('os').tmpdir();

var cssUrl = '';
var cssPath = path.join( tmpDir, 'style.css' );
request(cssUrl).pipe(fs.createWriteStream(cssPath)).on('close', function() {
  criticalcss.getRules(cssPath, function(err, output) {
    if (err) {
      throw new Error(err);
    } else {
      criticalcss.findCritical("", { rules: JSON.parse(output) }, function(err, output) {
        if (err) {
          throw new Error(err);
        } else {



Takes url or path to file, an options hash, and a callback function

Default values for options hash

  • width: 1200;
  • height: 900;
  • forceInclude: [];
  • rules: []; // REQUIRED
  • buffer: 800*1024;
  • ignoreConsole: false;


Type: Integer Default value: 1200

The width of the window being used for determining "above the fold"


Type: Integer Default value: 900

The height of the window being used for determining "above the fold"


Type: Array Default value: []

An array of selectors that should be included in the critical css no matter what. This is sometimes necessary with js-enhanced components.


Type: Array Default value: []

A JSON.stringifyd version of a CSSRuleList


Type: Integer Default value: 800*1024

Sets the maxBuffer for child_process.execFile in Node. Necessary for potential memory issues.


Type: Boolean Default value: false

Since criticalCSS handles output from STDOUT, it will also catch any outputs to console that are in the JavaScript on a page. If set to true, those will be silenced.


Takes a path to the CSS file and a callback function and returns a JSON.stringify'd subset of a CSSRuleList


Type: Boolean Default value: false

If you include @font-face declarations in your all.css file and set this flag to true in your options, criticalcss will include all the @font-face declarations that are required to satisfy font-family declarations in the criticalcss output.

Default values for options hash

  • buffer: 800*1024;


Sure, we got that.

  run  Run criticalcss

  --help                   Show help                                   [boolean]
  -u, --url                The url you would like to run criticalcss against
                                                             [string] [required]
  -f, --file               The local css file you're referencing for criticalcss
                                                             [string] [required]
  -w, --width              The width of your viewport            [default: 1200]
  -h, --height             The height of your viewport            [default: 900]
  -o, --output             The name of your output css file
                                             [string] [default: "dist/dist.css"]
  -b, --buffer             Specifies the largest amount of data allowed on
                           stdout or stderr - if this value is exceeded then the
                           child process is killed             [default: 819200]
  --force, --forceInclude  Sometimes selectors need to be forced into the
                           criticalcss                     [array] [default: []]
  -i, --ignoreConsole      Criticalcss will pick up console errors, set this to
                           ignore them                [boolean] [default: false]


Check out the tests!


In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint and test your code using Grunt.

Release History

  • v2.0.0 - Append the declarations from the original input CSS to selectors chosen by criticalCSS (includes vendor prefixes and other bits that the CSS object model misses).
  • v1.0.0 - Upgrade the underlying tool to Phantom 2.1
  • v0.6.0 - Add a CLI version
  • v0.5.0 - Add ignoreConsole option
  • v0.4.0 - Moved to using a local filename for getRules and moved to passing in a required Array of rules into the options hash on findCritical
  • v0.3.0 - Added forceInclude functionality. This allows the user to pass in an array of strings that are selectors. These selectors, if found in the CSS, will be treated as though they are above the fold and will therefore be automatically included.
  • v0.2.0 - Refactor
  • v0.1.0 - Hey, released this thing


Copyright (c) 2014 Scott Jehl/Jeffrey Lembeck/Filament Group Licensed under the MIT license.