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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|*]  Use `ie8` for compatibility mode, `*` for merge all (default)
-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 - ie8 for IE8 compatibility mode, * for merging all (default)
  • debug - set to true to get minification statistics under stats property (see test/custom-test.js for examples)

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

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.