A well-tested CSS minifier
Last updated 3 years ago by cnwhy .
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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 0.10+ (tested on CentOS, Ubuntu, OS X 10.6+, and Windows 7+) or io.js 3.0+

How to install clean-css?

npm install clean-css

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, [source-file, ...]

-h, --help                     output usage information
-v, --version                  output the version number
-b, --keep-line-breaks         Keep line breaks
-c, --compatibility [ie7|ie8]  Force compatibility mode (see Readme for advanced examples)
-d, --debug                    Shows debug information (minification time & compression efficiency)
-o, --output [output-file]     Use [output-file] as output instead of STDOUT
-r, --root [root-path]         Set a root path to which resolve absolute @import rules
-s, --skip-import              Disable @import processing
-t, --timeout [seconds]        Per connection timeout when fetching remote @imports (defaults to 5 seconds)
--rounding-precision [n]       Rounds to `N` decimal places. Defaults to 2. -1 disables rounding
--s0                           Remove all special comments, i.e. /*! comment */
--s1                           Remove all special comments but the first one
--semantic-merging             Enables unsafe mode by assuming BEM-like semantic stylesheets (warning, this may break your styling!)
--skip-advanced                Disable advanced optimizations - ruleset reordering & merging
--skip-aggressive-merging      Disable properties merging based on their order
--skip-import-from [rules]     Disable @import processing for specified rules
--skip-media-merging           Disable @media merging
--skip-rebase                  Disable URLs rebasing
--skip-restructuring           Disable restructuring optimizations
--skip-shorthand-compacting    Disable shorthand compacting
--source-map                   Enables building input's source map
--source-map-inline-sources    Enables inlining sources inside source maps


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 API?

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

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

  • advanced - set to false to disable advanced optimizations - selector & property merging, reduction, etc.
  • aggressiveMerging - set to false to disable aggressive merging of properties.
  • benchmark - turns on benchmarking mode measuring time spent on cleaning up (run npm run bench to see example)
  • compatibility - enables compatibility mode, see below for more examples
  • debug - set to true to get minification statistics under stats property (see test/custom-test.js for examples)
  • inliner - a hash of options for @import inliner, see test/protocol-imports-test.js for examples, or this comment for a proxy use case.
  • keepBreaks - whether to keep line breaks (default is false)
  • keepSpecialComments - * for keeping all (default), 1 for keeping first one only, 0 for removing all
  • mediaMerging - whether to merge @media at-rules (default is true)
  • processImport - whether to process @import rules
  • processImportFrom - a list of @import rules, can be ['all'] (default), ['local'], ['remote'], or a blacklisted path e.g. ['!fonts.googleapis.com']
  • rebase - set to false to skip URL rebasing
  • relativeTo - path to resolve relative @import rules and URLs
  • restructuring - set to false to disable restructuring in advanced optimizations
  • root - path to resolve absolute @import rules and rebase relative URLs
  • roundingPrecision - rounding precision; defaults to 2; -1 disables rounding
  • semanticMerging - set to true to enable semantic merging mode which assumes BEM-like content (default is false as it's highly likely this will break your stylesheets - use with caution!)
  • shorthandCompacting - set to false to skip shorthand compacting (default is true unless sourceMap is set when it's false)
  • sourceMap - exposes source map under sourceMap property, e.g. new CleanCSS().minify(source).sourceMap (default is false) If input styles are a product of CSS preprocessor (Less, Sass) an input source map can be passed as a string.
  • sourceMapInlineSources - set to true to inline sources inside a source map's sourcesContent field (defaults to false) It is also required to process inlined sources from input source maps.
  • target - path to a folder or an output file to which rebase all URLs

The output of minify method (or the 2nd argument to passed callback) is a hash containing the following fields:

  • styles - optimized output CSS as a string
  • sourceMap - output source map (if requested with sourceMap option)
  • errors - a list of errors raised
  • warnings - a list of warnings raised
  • stats - a hash of statistic information (if requested with debug option):
    • originalSize - original content size (after import inlining)
    • minifiedSize - optimized content size
    • timeSpent - time spent on optimizations
    • efficiency - a ratio of output size to input size (e.g. 25% if content was reduced from 100 bytes to 75 bytes)

How to make sure remote @imports are processed correctly?

In order to inline remote @import statements you need to provide a callback to minify method, e.g.:

var CleanCSS = require('clean-css');
var source = '@import url(http://path/to/remote/styles);';
new CleanCSS().minify(source, function (errors, minified) {
  // minified.styles

This is due to a fact, that, while local files can be read synchronously, remote resources can only be processed asynchronously. If you don't provide a callback, then remote @imports will be left intact.

How to use clean-css with build tools?

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 browserify to create the browser-ready clean-css version
  • npm run check to check JS sources with JSHint
  • npm test for the test suite

How to contribute to clean-css?


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.

How to generate source maps?

Source maps are supported since version 3.0.

Additionally to mapping original CSS files, clean-css also supports input source maps, so minified styles can be mapped into their Less or Sass sources directly.

Source maps are generated using source-map module from Mozilla.

Using CLI

To generate a source map, use --source-map switch, e.g.:

cleancss --source-map --output styles.min.css styles.css

Name of the output file is required, so a map file, named by adding .map suffix to output file name, can be created (styles.min.css.map in this case).

Using API

To generate a source map, use sourceMap: true option, e.g.:

new CleanCSS({ sourceMap: true, target: pathToOutputDirectory })
  .minify(source, function (minified) {
    // access minified.sourceMap for SourceMapGenerator object
    // see https://github.com/mozilla/source-map/#sourcemapgenerator for more details
    // see https://github.com/jakubpawlowicz/clean-css/blob/master/bin/cleancss#L114 on how it's used in clean-css' CLI

Using API you can also pass an input source map directly:

new CleanCSS({ sourceMap: inputSourceMapAsString, target: pathToOutputDirectory })
  .minify(source, function (minified) {
    // access minified.sourceMap to access SourceMapGenerator object
    // see https://github.com/mozilla/source-map/#sourcemapgenerator for more details
    // see https://github.com/jakubpawlowicz/clean-css/blob/master/bin/cleancss#L114 on how it's used in clean-css' CLI

Or even multiple input source maps at once (available since version 3.1):

new CleanCSS({ sourceMap: true, target: pathToOutputDirectory }).minify({
  'path/to/source/1': {
    styles: '...styles...',
    sourceMap: '...source-map...'
  'path/to/source/2': {
    styles: '...styles...',
    sourceMap: '...source-map...'
}, function (minified) {
  // access minified.sourceMap as above

How to minify multiple files with API?

Passing an array

new CleanCSS().minify(['path/to/file/one', 'path/to/file/two']);

Passing a hash

new CleanCSS().minify({
  'path/to/file/one': {
    styles: 'contents of file one'
  'path/to/file/two': {
    styles: 'contents of file two'

How to set a compatibility mode?

Compatibility settings are controlled by --compatibility switch (CLI) and compatibility option (library mode).

In both modes the following values are allowed:

  • 'ie7' - Internet Explorer 7 compatibility mode
  • 'ie8' - Internet Explorer 8 compatibility mode
  • '' or '*' (default) - Internet Explorer 9+ compatibility mode

Since clean-css 3 a fine grained control is available over those settings, with the following options available:

  • '[+-]colors.opacity' - - turn on (+) / off (-) rgba() / hsla() declarations removal
  • '[+-]properties.backgroundClipMerging' - turn on / off background-clip merging into shorthand
  • '[+-]properties.backgroundOriginMerging' - turn on / off background-origin merging into shorthand
  • '[+-]properties.backgroundSizeMerging' - turn on / off background-size merging into shorthand
  • '[+-]properties.colors' - turn on / off any color optimizations
  • '[+-]properties.ieBangHack' - turn on / off IE bang hack removal
  • '[+-]properties.iePrefixHack' - turn on / off IE prefix hack removal
  • '[+-]properties.ieSuffixHack' - turn on / off IE suffix hack removal
  • '[+-]properties.merging' - turn on / off property merging based on understandability
  • '[+-]properties.spaceAfterClosingBrace' - turn on / off removing space after closing brace - url() no-repeat into url()no-repeat
  • '[+-]properties.urlQuotes' - turn on / off url() quoting
  • '[+-]properties.zeroUnits' - turn on / off units removal after a 0 value
  • '[+-]selectors.adjacentSpace' - turn on / off extra space before nav element
  • '[+-]selectors.ie7Hack' - turn on / off IE7 selector hack removal (*+html...)
  • '[+-]selectors.special' - a regular expression with all special, unmergeable selectors (leave it empty unless you know what you are doing)
  • '[+-]units.ch' - turn on / off treating ch as a proper unit
  • '[+-]units.in' - turn on / off treating in as a proper unit
  • '[+-]units.pc' - turn on / off treating pc as a proper unit
  • '[+-]units.pt' - turn on / off treating pt as a proper unit
  • '[+-]units.rem' - turn on / off treating rem as a proper unit
  • '[+-]units.vh' - turn on / off treating vh as a proper unit
  • '[+-]units.vm' - turn on / off treating vm as a proper unit
  • '[+-]units.vmax' - turn on / off treating vmax as a proper unit
  • '[+-]units.vmin' - turn on / off treating vmin as a proper unit
  • '[+-]units.vm' - turn on / off treating vm as a proper unit

For example, using --compatibility 'ie8,+units.rem' will ensure IE8 compatibility while enabling rem units so the following style margin:0px 0rem can be shortened to margin:0, while in pure IE8 mode it can't be.

To pass a single off (-) switch in CLI please use the following syntax --compatibility *,-units.rem.

In library mode you can also pass compatibility as a hash of options.

What advanced optimizations are applied?

All advanced optimizations are dispatched here, and this is what they do:

  • recursivelyOptimizeBlocks - does all the following operations on a block (think @media or @keyframe at-rules);
  • recursivelyOptimizeProperties - optimizes properties in rulesets and "flat at-rules" (like @font-face) by splitting them into components (e.g. margin into margin-(*)), optimizing, and rebuilding them back. You may want to use shorthandCompacting option to control whether you want to turn multiple (long-hand) properties into a shorthand ones;
  • removeDuplicates - gets rid of duplicate rulesets with exactly the same set of properties (think of including the same Sass / Less partial twice for no good reason);
  • mergeAdjacent - merges adjacent rulesets with the same selector or rules;
  • reduceNonAdjacent - identifies which properties are overridden in same-selector non-adjacent rulesets, and removes them;
  • mergeNonAdjacentBySelector - identifies same-selector non-adjacent rulesets which can be moved (!) to be merged, requires all intermediate rulesets to not redefine the moved properties, or if redefined to be either more coarse grained (e.g. margin vs margin-top) or have the same value;
  • mergeNonAdjacentByBody - same as the one above but for same-rules non-adjacent rulesets;
  • restructure - tries to reorganize different-selector different-rules rulesets so they take less space, e.g. .one{padding:0}.two{margin:0}.one{margin-bottom:3px} into .two{margin:0}.one{padding:0;margin-bottom:3px};
  • removeDuplicateMediaQueries - removes duplicated @media at-rules;
  • mergeMediaQueries - merges non-adjacent @media at-rules by same rules as mergeNonAdjacentBy* above;

Acknowledgments (sorted alphabetically)

  • Anthony Barre (@abarre) for improvements to @import processing, namely introducing the --skip-import / processImport options.
  • Simon Altschuler (@altschuler) for fixing @import processing inside comments.
  • 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.
  • Luke Page (@lukeapage) for suggestions and testing the source maps feature. Plus everyone else involved in #125 for pushing it forward.
  • Timur Kristóf (@Venemo) for an outstanding contribution of advanced property optimizer for 2.2 release.
  • Vincent Voyer (@vvo) for a patch with better empty element regex and for inspiring us to do many performance improvements in 0.4 release.
  • @XhmikosR for suggesting new features (option to remove special comments and strip out URLs quotation) and pointing out numerous improvements (JSHint, media queries).


Clean-css is released under the MIT License.

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