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What is clean-css?

Clean-css is a fast and efficient Node.js library for minifying CSS files.

According to tests it is one of the best available.


What are the requirements?

Node.js 0.8.0+ (tested on CentOS, Ubuntu, OS X 10.6+, and Windows 7+)

How to install clean-css?

npm install clean-css

How to upgrade clean-css from 1.x to 2.x?

Command-line interface (CLI)

npm update clean-css

or point package.json to version 2.x. That's it!

Node.js module

Update clean-css as for CLI above. Then change your JavaScript code from:

var minimized = CleanCSS.process(source, options);


var minimized = new CleanCSS(options).minify(source);

And you are done.

How to use clean-css CLI?

Clean-css accepts the following command line arguments (please make sure you use <source-file> as the very last argument to avoid potential issues):

cleancss [options] <source-file>

-h, --help                      Output usage information
-v, --version                   Output the version number
-b, --keep-line-breaks          Keep line breaks
--s0                            Remove all special comments, i.e. /! comment /
--s1                            Remove all special comments but the first one
-r, --root [root-path]          A root path to which resolve absolute @import rules
                                and rebase relative URLs
-o, --output [output-file]      Use [output-file] as output instead of STDOUT
-s, --skip-import               Disable @import processing
--skip-rebase                   Disable URLs rebasing
--skip-advanced                 Disable advanced optimizations - selector & property merging,
                                reduction, etc.
--selectors-merge-mode [ie8|*]  DEPRECATED: Use --compatibility switch
-c, --compatibility [ie7|ie8]   Force compatibility mode
-d, --debug                     Shows debug information (minification time & compression efficiency)


To minify a public.css file into public-min.css do:

cleancss -o public-min.css public.css

To minify the same public.css into the standard output skip the -o parameter:

cleancss public.css

More likely you would like to concatenate a couple of files. If you are on a Unix-like system:

cat one.css two.css three.css | cleancss -o merged-and-minified.css

On Windows:

type one.css two.css three.css | cleancss -o merged-and-minified.css

Or even gzip the result at once:

cat one.css two.css three.css | cleancss | gzip -9 -c > merged-minified-and-gzipped.css.gz

How to use clean-css programmatically?

var CleanCSS = require('clean-css');
var source = 'a{font-weight:bold;}';
var minimized = new CleanCSS().minify(source);

CleanCSS constructor accepts a hash as a parameter, i.e., new CleanCSS(options).minify(source) with the following options available:

  • keepSpecialComments - * for keeping all (default), 1 for keeping first one only, 0 for removing all
  • keepBreaks - whether to keep line breaks (default is false)
  • benchmark - turns on benchmarking mode measuring time spent on cleaning up (run npm run bench to see example)
  • root - path to resolve absolute @import rules and rebase relative URLs
  • relativeTo - path with which to resolve relative @import rules and URLs
  • processImport - whether to process @import rules
  • noRebase - whether to skip URLs rebasing
  • noAdvanced - set to true to disable advanced optimizations - selector & property merging, reduction, etc.
  • selectorsMergeMode - DEPRECATED: Use compatibility option
  • compatibility - Force compatibility mode to ie7 or ie8. Defaults to not set.
  • debug - set to true to get minification statistics under stats property (see test/custom-test.js for examples)

How to use clean-css with Grunt?

See grunt-contrib-cssmin plugin.

What are the clean-css' dev commands?

First clone the source, then run:

  • npm run bench for clean-css benchmarks (see test/bench.js for details)
  • npm run check to check JS sources with JSHint
  • npm test for the test suite

How to contribute to clean-css?

  1. Fork it.
  2. Add test(s) veryfying the problem.
  3. Fix the problem.
  4. Make sure all tests still pass (npm test).
  5. Make sure your code doesn't break style rules (npm run check).
  6. Send a PR.

If you wonder where to add tests, go for:

  • test/unit-test.js if it's a simple scenario
  • test/data/... if it's a complex scenario (just add two files, input and expected output)
  • test/binary-test.js if it's related to bin/cleancss binary
  • test/module-test.js if it's related to importing clean-css as a module
  • test/protocol-imports-test.js if it fixes anything related to protocol @imports

Tips & Tricks

How to preserve a comment block?

Use the /*! notation instead of the standard one /*:

  Important comments included in minified output.

How to rebase relative image URLs

Clean-css will handle it automatically for you (since version 1.1) in the following cases:

  • When using the CLI:
    1. Use an output path via -o/--output to rebase URLs as relative to the output file.
    2. Use a root path via -r/--root to rebase URLs as absolute from the given root path.
    3. If you specify both then -r/--root takes precendence.
  • When using clean-css as a library:
    1. Use a combination of relativeTo and target options for relative rebase (same as 1 in CLI).
    2. Use a combination of relativeTo and root options for absolute rebase (same as 2 in CLI).
    3. root takes precendence over target as in CLI.


  • Vincent Voyer (@vvo) for a patch with better empty element regex and for inspiring us to do many performance improvements in 0.4 release.
  • Isaac (@facelessuser) for pointing out a flaw in clean-css' stateless mode.
  • Jan Michael Alonzo (@jmalonzo) for a patch removing node.js' old sys package.
  • @XhmikosR for suggesting new features (option to remove special comments and strip out URLs quotation) and pointing out numerous improvements (JSHint, media queries).
  • Anthony Barre (@abarre) for improvements to @import processing, namely introducing the --skip-import / processImport options.
  • Simon Altschuler (@altschuler) for fixing @import processing inside comments.


Clean-css is released under the MIT License.