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Plugin based CSS Compressor.

  • Full Featured Demo and Rule Documentation
  • Extensive logging of each compression, including position/selection of text in the stylesheet
  • Entirely customizable, can turn individual compressions on/off with no dependencies.
  • Writing custom compressions are relatively easy.

Build Status


$ npm install csscompressor 

CSSCompressor may also be run in a browser environment.

<script type='text/javascript' src='CSSCompressor.js'></script>


var CSSCompressor = require( 'csscompressor' );
// Compression with default options 
CSSCompressor.compress( css );

Options & Modes

Modes are a way to use a predefined set of rule compressions. Each compressor starts off in the 'default' mode, and compressions can be turned off/on through the options object of the compress function, or updating the settings object.

var compressor = new CSSCompressor( CSSCompressor.MODE_DEFAULT );
compressor.compress( css );
// Pass an object of compression options to update the default set 
compressor.compress( css, {
    'RGB to Hex': true,
    'Hex to Color': false
// Or update the settings object directly. 
    'RGB to Hex': true,
    'Hex to Color': false

Take a look at the rules section of the demo to get a list of all the possible options.


To install the build script, add the global flag when installing through npm

$ npm install -g csscompressor
$ cssc styles.css > styles.min.css
// Or you can pipe your sheet
$ cat styles.css | cssc > styles.min.css

By default, the script reads the files in order they are passed, concatenates them, compresses them, then sends it to stdout. For the most part, CSSCompressor is meant to belong in build scripts, but there are a few options that can be applied to the cli script

  • --mode=name: Assigns a mode to the compressor object
  • --on=rule-name: Turns rule compression on
  • --off=rule-name: Turns rule compression off
  • --format=none: Level of output format, defaulted to none
  • --output=/path/to/output.min.css: Specifies output path of compression result. If left undefined, output is sent to stdout.


Adding new compression rules is fairly straightforward, to get started clone the repo and run initialization.

git clone
cd CSSCompressor/
make init

This will setup all third party modules and generated files. Once setup, just follow the checklist below before completion.

  1. Only add 1 compression rule per commit.
  2. Create a test file (tests/test-[name of rule].js) with a few expected tests. Example
  3. Create a new rule file (lib/rules/[name of rule].js) with exactly one rule. Follow the comment notation of other rules. Example
  4. Add rule file to the build libs configuration.
  5. Run make test. All tests must pass.

Adding Compression Rules

CSSCompressor is built on modular based compressions. Each compression option has it's own function that finds the exact parts it wants to alter, and makes changes. With that, there are 4 types of custom rules that may be applied: addRule, addRuleBlock, addRuleSheet, and addValue. If you haven't yet, please go look at the CSSTree document before reading further to get an idea of what a branch looks like.


addRule is used for individual property/value compressions. For example, converting the color value 'black' to it's shorter hex code alternative '#000':

CSSCompressor.addRule( 'Special Black to Hex Converter', function( rule, branch, compressor ) {
    if ( == 'color' &&[ 0 ] == 'black' ) {[ 0 ] = '#000';


addRuleBlock is used for rule set compressions. A full branch is passed to the callback to be rendered for combination style compressions. For example, removing all color properties in a div block

CSSCompressor.addRuleBlock( 'Remove All Color Properties in Divs', function( branch, compressor ) {
    if ( ! branch.selector || ! branch.selector.exec( /div$/i ) || ! branch.rules ) {
    for ( var i = 0, rule; i < branch.rules.length; i++ ) {
        rule = branch.rules[ i ];
        if ( == 'color' ) {
            branch.rules.splice( i, 1 );


addRuleSheet is used for full stylesheet compressions. The callback is passed the entire stylesheet AST in the branches array for inspection and compression. For example, to remove all comments:

CSSCompressor.addRuleSheet( 'Remove Comments', function( branches, compressor ) {
    for ( var i = 0, branch; ++< branches.length; i++ ) {
        branch = branches[ i ];
        if ( branch.comment ) {
            branches.splice( i, 1 );


addValue is used for specific string value compressions. It works a little different in that there are no rules or blocks attached to the string, only a possible position. An example would be compressing color strings inside gradient values. This is the only compression that requires a return value

CSSCompressor.addValue( 'Removing Leading Zeros on Numerics', function( value, position, compressor ) {
    var m = /^0+(\d+[a-z]{2})$/.exec( value );
    if ( m ) {
        return m[ 1 ];


Every branch and rule in a CSSTree comes with a position object that contains a ton of useful information about it's parent object. In CSSCompressor's demo site, the logging function is used to mark the original position in the stylesheet for that compression.

compressor.log( [ key, ] msg, position );

The log method takes 3 parameters, an optional string name of the compression function used, a string message describing what was changed, and the position object of the affected rule/branch. The last parameter may optionally be an array of position objects if multiple branches/rules are affected. Taking the color example from above and adding a log line to it:

CSSCompressor.addRule( 'Special Black to Hex Converter', function( rule, branch, compressor ) {
    if ( == 'color' &&[ 0 ] == 'black' ) {[ 0 ] = '#000';
        compressor.log( "Converting black to it's hex alternative", rule.position );

Build Process

This compressor can also be used as part of a build process to force a cache bump on resources. The following example shows to to look for all url's and attaches the current unix timestamp to the query string of the url.

var now =, rurl = /^url\(.*?\)$/;
CSSCompressor.addValue( 'Resource Refresh', function( value, position, compressor ) {
    if ( rurl.exec( value ) ) {
        var query = value.indexOf( '?' ) > -1 ? '&d=' + now : '&d=' + now;
        // "url(img/phone.png)" -> "url(img/phone.png?d=1366303414438)" 
        return value.substr( 0, value.length - 2 ) + query + ')';


The MIT License
Copyright (c) 2013 Corey Hart
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.