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1. Introduction

CSSO (CSS Optimizer) is a CSS minimizer unlike others. In addition to usual minification techniques it can perform structural optimization of CSS files, resulting in smaller file size compared to other minifiers.

This document describes installation and usage of CSSO. If you want to learn more about the inner workings of CSSO, please consult the [manual] (

Please report issues on [Github] (

For feedback, suggestions, etc. write to

2. Installation

2.1. Prerequisites

  • for browser use: any OS and a modern web browser
  • for command line use: Linux / Mac OS X / any OS with working Node.js

2.2. Install using git


To install:

  • run git clone git://

2.3. Install using npm


To install (global):

  • run npm install csso -g

To update:

  • run npm update csso

To uninstall:

  • run npm uninstall csso

3. Usage

3.1. In the browser

Open web/csso.html or in your browser.

CSSO is not guaranteed to work in browsers. Preferred way to use this tool is to run it from the command line or via npm modules.

3.2. As an npm module

Sample (test.js):

var csso = require('csso'),
    css = '.test, .test { color: rgb(255, 255, 255) }';


Output (> node test.js):


Use csso.justDoIt(css, true) to turn structure minimization off.

3.3. From the command line

Run bin/csso (when installed from git), you will need to have nodejs 0.4.x installed —

Run csso (when installed from npm).


    shows usage information
csso <filename>
    minimizes the CSS in <filename> and outputs the result to stdout
csso <in_filename> <out_filename>
csso -i <in_filename> -o <out_filename>
csso --input <in_filename> --output <out_filename>
    minimizes the CSS in <in_filename> and outputs the result to <out_filename>
csso -off
csso --restructure-off
    turns structure minimization off
csso -h
csso --help
    shows usage information
csso -v
csso --version
    shows the version number


$ echo ".test { color: red; color: green }" > test.css
$ csso test.css

4. Minification (in a nutshell)

Safe transformations:

  • Removal of whitespace
  • Removal of trailing ;
  • Removal of comments
  • Removal of invalid @charset и @import declarations
  • Minification of color properties
  • Minification of 0
  • Minification of multi-line strings
  • Minification of the font-weight property

Structural optimizations:

  • Merging blocks with identical selectors
  • Merging blocks with identical properties
  • Removal of overridden properties
  • Removal of overridden shorthand properties
  • Removal of repeating selectors
  • Partial merging of blocks
  • Partial splitting of blocks
  • Removal of empty ruleset and at-rule
  • Minification of margin and padding properties

The minification techniques are described in detail in the manual.

5. Authors

6. And finally

  • CSSO is licensed under MIT