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clean-css is a fast and efficient CSS optimizer for Node.js platform and any modern browser.

According to tests it is one of the best available.

Table of Contents

Node.js version support

clean-css requires Node.js 4.0+ (tested on Linux, OS X, and Windows)


npm install clean-css


var CleanCSS = require('clean-css');
var input = 'a{font-weight:bold;}';
var options = { /* options */ };
var output = new CleanCSS(options).minify(input);

Important: 4.0 breaking changes

clean-css 4.0 introduces some breaking changes:

  • API and CLI interfaces are split, so API stays in this repository while CLI moves to clean-css-cli;
  • root, relativeTo, and target options are replaced by a single rebaseTo option - this means that rebasing URLs and import inlining is much simpler but may not be (YMMV) as powerful as in 3.x;
  • debug option is gone as stats are always provided in output object under stats property;
  • roundingPrecision is disabled by default;
  • roundingPrecision applies to all units now, not only px as in 3.x;
  • processImport and processImportFrom are merged into inline option which defaults to local. Remote @import rules are NOT inlined by default anymore;
  • splits inliner: { request: ..., timeout: ... } option into inlineRequest and inlineTimeout options;
  • remote resources without a protocol, e.g. //, are not inlined anymore;
  • changes default Internet Explorer compatibility from 9+ to 10+, to revert the old default use { compatibility: 'ie9' } flag;
  • renames keepSpecialComments to specialComments;
  • moves roundingPrecision and specialComments to level 1 optimizations options, see examples;
  • moves mediaMerging, restructuring, semanticMerging, and shorthandCompacting to level 2 optimizations options, see examples below;
  • renames shorthandCompacting option to mergeIntoShorthands;
  • level 1 optimizations are the new default, up to 3.x it was level 2;
  • keepBreaks option is replaced with { format: 'keep-breaks' } to ease transition;
  • sourceMap option has to be a boolean from now on - to specify an input source map pass it a 2nd argument to minify method or via a hash instead;
  • aggressiveMerging option is removed as aggressive merging is replaced by smarter override merging.

Constructor options

clean-css constructor accepts a hash as a parameter with the following options available:

  • compatibility - controls compatibility mode used; defaults to ie10+; see compatibility modes for examples;
  • format - controls output CSS formatting; defaults to false; see formatting options for examples;
  • inline - controls @import inlining rules; defaults to 'local'; see inlining options for examples;
  • inlineRequest - controls extra options for inlining remote @import rules, can be any of HTTP(S) request options;
  • inlineTimeout - controls number of milliseconds after which inlining a remote @import fails; defaults to 5000;
  • level - controls optimization level used; defaults to 1; see optimization levels for examples;
  • rebase - controls URL rebasing; defaults to true;
  • rebaseTo - controls a directory to which all URLs are rebased, most likely the directory under which the output file will live; defaults to the current directory;
  • returnPromise - controls whether minify method returns a Promise object or not; defaults to false; see promise interface for examples;
  • sourceMap - controls whether an output source map is built; defaults to false;
  • sourceMapInlineSources - controls embedding sources inside a source map's sourcesContent field; defaults to false.

Compatibility modes

There is a certain number of compatibility mode shortcuts, namely:

  • new CleanCSS({ compatibility: '*' }) (default) - Internet Explorer 10+ compatibility mode
  • new CleanCSS({ compatibility: 'ie9' }) - Internet Explorer 9+ compatibility mode
  • new CleanCSS({ compatibility: 'ie8' }) - Internet Explorer 8+ compatibility mode
  • new CleanCSS({ compatibility: 'ie7' }) - Internet Explorer 7+ compatibility mode

Each of these modes is an alias to a fine grained configuration, with the following options available:

new CleanCSS({
  compatibility: {
    colors: {
      opacity: true // controls `rgba()` / `hsla()` color support 
    properties: {
      backgroundClipMerging: true, // controls background-clip merging into shorthand 
      backgroundOriginMerging: true, // controls background-origin merging into shorthand 
      backgroundSizeMerging: true, // controls background-size merging into shorthand 
      colors: true, // controls color optimizations 
      ieBangHack: false, // controls keeping IE bang hack 
      ieFilters: false, // controls keeping IE `filter` / `-ms-filter` 
      iePrefixHack: false, // controls keeping IE prefix hack 
      ieSuffixHack: false, // controls keeping IE suffix hack 
      merging: true, // controls property merging based on understandability 
      shorterLengthUnits: false, // controls shortening pixel units into `pc`, `pt`, or `in` units 
      spaceAfterClosingBrace: true, // controls keeping space after closing brace - `url() no-repeat` into `url()no-repeat` 
      urlQuotes: false, // controls keeping quoting inside `url()` 
      zeroUnits: true // controls removal of units `0` value 
    selectors: {
      adjacentSpace: false, // controls extra space before `nav` element 
      ie7Hack: true, // controls removal of IE7 selector hacks, e.g. `*+html...` 
      mergeablePseudoClasses: [':active', ...], // controls a whitelist of mergeable pseudo classes 
      mergeablePseudoElements: ['::after', ...] // controls a whitelist of mergeable pseudo elements 
    units: {
      ch: true, // controls treating `ch` as a supported unit 
      in: true, // controls treating `in` as a supported unit 
      pc: true, // controls treating `pc` as a supported unit 
      pt: true, // controls treating `pt` as a supported unit 
      rem: true, // controls treating `rem` as a supported unit 
      vh: true, // controls treating `vh` as a supported unit 
      vm: true, // controls treating `vm` as a supported unit 
      vmax: true, // controls treating `vmax` as a supported unit 
      vmin: true // controls treating `vmin` as a supported unit 

You can also use a string when setting a compatibility mode, e.g.

new CleanCSS({
  compatibility: 'ie9,-properties.merging' // sets compatibility to IE9 mode with disabled property merging 

Formatting options

The format option accept the following options:

new CleanCSS({
  format: {
    breaks: { // controls where to insert breaks 
      afterAtRule: false, // controls if a line break comes after an at-rule; e.g. `@charset`; defaults to `false` 
      afterBlockBegins: false, // controls if a line break comes after a block begins; e.g. `@media`; defaults to `false` 
      afterBlockEnds: false, // controls if a line break comes after a block ends, defaults to `false` 
      afterComment: false, // controls if a line break comes after a comment; defaults to `false` 
      afterProperty: false, // controls if a line break comes after a property; defaults to `false` 
      afterRuleBegins: false, // controls if a line break comes after a rule begins; defaults to `false` 
      afterRuleEnds: false, // controls if a line break comes after a rule ends; defaults to `false` 
      beforeBlockEnds: false, // controls if a line break comes before a block ends; defaults to `false` 
      betweenSelectors: false // controls if a line break comes between selectors; defaults to `false` 
    indentBy: 0, // controls number of characters to indent with; defaults to `0` 
    indentWith: 'space', // controls a character to indent with, can be `'space'` or `'tab'`; defaults to `'space'` 
    spaces: { // controls where to insert spaces 
      aroundSelectorRelation: false, // controls if spaces come around selector relations; e.g. `div > a`; defaults to `false` 
      beforeBlockBegins: false, // controls if a space comes before a block begins; e.g. `.block {`; defaults to `false` 
      beforeValue: false // controls if a space comes before a value; e.g. `width: 1rem`; defaults to `false` 
    wrapAt: false // controls maximum line length; defaults to `false` 

Inlining options

inline option whitelists which @import rules will be processed, e.g.

new CleanCSS({
  inline: ['local'] // default 
new CleanCSS({
  inline: ['all'] // same as ['local', 'remote'] 
new CleanCSS({
  inline: ['local', '']
new CleanCSS({
  inline: ['local', 'remote', '!']

Optimization levels

The level option can be either 0, 1 (default), or 2, e.g.

new CleanCSS({
  level: 2

or a fine-grained configuration given via a hash.

Please note that level 1 optimization options are generally safe while level 2 optimizations should be safe for most users.

Level 0 optimizations

Level 0 optimizations simply means "no optimizations". Use it when you'd like to inline imports and / or rebase URLs but skip everything else.

Level 1 optimizations

Level 1 optimizations (default) operate on single properties only, e.g. can remove units when not required, turn rgb colors to a shorter hex representation, remove comments, etc

Here is a full list of available options:

new CleanCSS({
  level: {
    1: {
      cleanupCharsets: true, // controls `@charset` moving to the front of a stylesheet; defaults to `true` 
      normalizeUrls: true, // controls URL normalization; defaults to `true` 
      optimizeBackground: true, // controls `background` property optimizatons; defaults to `true` 
      optimizeBorderRadius: true, // controls `border-radius` property optimizatons; defaults to `true` 
      optimizeFilter: true, // controls `filter` property optimizatons; defaults to `true` 
      optimizeFont: true, // ontrols `font` property optimizatons; defaults to `true` 
      optimizeFontWeight: true, // controls `font-weight` property optimizatons; defaults to `true` 
      optimizeOutline: true, // controls `outline` property optimizatons; defaults to `true` 
      removeNegativePaddings: true, // controls removing negative paddings; defaults to `true` 
      removeQuotes: true, // controls removing quotes when unnecessary; defaults to `true` 
      removeWhitespace: true, // controls removing unused whitespace; defaults to `true` 
      replaceMultipleZeros: true, // contols removing redundant zeros; defaults to `true` 
      replaceTimeUnits: true, // controls replacing time units with shorter values; defaults to `true` 
      replaceZeroUnits: true, // controls replacing zero values with units; defaults to `true` 
      roundingPrecision: false, // rounds pixel values to `N` decimal places; `false` disables rounding; defaults to `false` 
      selectorsSortingMethod: 'standard', // denotes selector sorting method; can be `natural` or `standard`; defaults to `standard` 
      specialComments: 'all', // denotes a number of /*! ... */ comments preserved; defaults to `all` 
      tidyAtRules: true, // controls at-rules (e.g. `@charset`, `@import`) optimizing; defaults to `true` 
      tidyBlockScopes: true, // controls block scopes (e.g. `@media`) optimizing; defaults to `true` 
      tidySelectors: true, // controls selectors optimizing; defaults to `true`, 
      transform: function () {} // defines a callback for fine-grained property optimization; defaults to no-op 

There is an all shortcut for toggling all options at the same time, e.g.

new CleanCSS({
  level: {
    1: {
      all: false, // set all values to `false` 
      tidySelectors: true // turns on optimizing selectors 

Level 2 optimizations

Level 2 optimizations operate at rules or multiple properties level, e.g. can remove duplicate rules, remove properties redefined further down a stylesheet, or restructure rules by moving them around.

Please note that if level 2 optimizations are turned on then, unless explicitely disabled, level 1 optimizations are applied as well.

Here is a full list of available options:

new CleanCSS({
  level: {
    2: {
      mergeAdjacentRules: true, // controls adjacent rules merging; defaults to true 
      mergeIntoShorthands: true, // controls merging properties into shorthands; defaults to true 
      mergeMedia: true, // controls `@media` merging; defaults to true 
      mergeNonAdjacentRules: true, // controls non-adjacent rule merging; defaults to true 
      mergeSemantically: false, // controls semantic merging; defaults to false 
      overrideProperties: true, // controls property overriding based on understandability; defaults to true 
      reduceNonAdjacentRules: true, // controls non-adjacent rule reducing; defaults to true 
      removeDuplicateFontRules: true, // controls duplicate `@font-face` removing; defaults to true 
      removeDuplicateMediaBlocks: true, // controls duplicate `@media` removing; defaults to true 
      removeDuplicateRules: true, // controls duplicate rules removing; defaults to true 
      restructureRules: false // controls rule restructuring; defaults to false 

There is an all shortcut for toggling all options at the same time, e.g.

new CleanCSS({
  level: {
    2: {
      all: false, // sets all values to `false` 
      removeDuplicateRules: true // turns on removing duplicate rules 

Minify method

Once configured clean-css provides a minify method to optimize a given CSS, e.g.

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

The output of the minify method is a hash with following fields:

console.log(output.styles); // optimized output CSS as a string 
console.log(output.sourceMap); // output source map if requested with `sourceMap` option 
console.log(output.errors); // a list of errors raised 
console.log(output.warnings); // a list of warnings raised 
console.log(output.stats.originalSize); // original content size after import inlining 
console.log(output.stats.minifiedSize); // optimized content size 
console.log(output.stats.timeSpent); // time spent on optimizations in milliseconds 
console.log(output.stats.efficiency); // a ratio of output size to input size (e.g. 25% if content was reduced from 100 bytes to 75 bytes) 

The minify method also accepts an input source map, e.g.

var output = new CleanCSS(options).minify(source, inputSourceMap);

or a callback invoked when optimizations are finished, e.g.

new CleanCSS(options).minify(source, function (error, output) {
  // `output` is the same as in the synchronous call above 

Promise interface

If you prefer clean-css to return a Promise object then you need to explicitely ask for it, e.g.

new CleanCSS({ returnPromise: true })
  .then(function (output) { console.log(output.styles); })
  .catch(function (error) { // deal with errors }); 


How to optimize multiple files?

It can be done either by passing an array of paths, or, when sources are already available, a hash:

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

Important note - any @import rules already present in the hash will be resolved in memory.

How to process remote @imports correctly?

In order to inline remote @import statements you need to provide a callback to minify method as fetching remote assets is an asynchronous operation, e.g.:

var source = '@import url(;';
new CleanCSS({ inline: ['remote'] }).minify(source, function (error, output) {
  // output.styles 

If you don't provide a callback, then remote @imports will be left as is.

How to apply arbitrary transformations to CSS properties?

If clean-css doesn't perform a particular property optimization, you can use transform callback to apply it:

var source = '.block{background-image:url(/path/to/image.png)}';
var output = new CleanCSS({
  level: {
    1: {
      transform: function (propertyName, propertyValue) {
        if (propertyName == 'background-image' && propertyValue.indexOf('/path/to') > -1) {
          return propertyValue.replace('/path/to', '../valid/path/to');
console.log(output.styles); # => .block{background-image:url(../valid/path/to/image.png)}

Note: returning false from transform callback will drop a property.

How to specify a custom rounding precision?

The level 1 roundingPrecision optimization option accept a string with per-unit rounding precision settings, e.g.

new CleanCSS({
  level: {
    1: {
      roundingPrecision: 'all=3,px=5'

which sets all units rounding precision to 3 digits except px unit precision of 5 digits.

How to preserve a comment block?

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

  Important comments included in optimized output.

How to rebase relative image URLs?

clean-css will handle it automatically for you in the following cases:

  • when full paths to input files are passed in as options;
  • when correct paths are passed in via a hash;
  • when rebaseTo is used with any of above two.

How to work with source maps?

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

new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory })
  .minify(source, function (error, output) {
    // access output.sourceMap for SourceMapGenerator object 
    // see for more details 

You can also pass an input source map directly as a 2nd argument to minify method:

new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory })
  .minify(source, inputSourceMap, function (error, output) {
    // access output.sourceMap to access SourceMapGenerator object 
    // see for more details 

or even multiple input source maps at once:

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

How to apply level 1 & 2 optimizations at the same time?

Using the hash configuration specifying both optimization levels, e.g.

new CleanCSS({
  level: {
    1: {
      all: true,
      normalizeUrls: false
    2: {
      restructureRules: true

will apply level 1 optimizations, except url normalization, and default level 2 optimizations with rule restructuring.

What level 2 optimizations do?

All level 2 optimizations are dispatched here, and this is what they do:

  • recursivelyOptimizeBlocks - does all the following operations on a nested block, like @media or @keyframe;
  • recursivelyOptimizeProperties - optimizes properties in rulesets and flat at-rules, like @font-face, by splitting them into components (e.g. margin into margin-(bottom|left|right|top)), optimizing, and restoring them back. You may want to use mergeIntoShorthands option to control whether you want to turn multiple components into shorthands;
  • removeDuplicates - gets rid of duplicate rulesets with exactly the same set of properties, e.g. when including a 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 have the same value;
  • mergeNonAdjacentByBody - same as the one above but for same-selector 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};
  • removeDuplicateFontAtRules - removes duplicated @font-face rules;
  • removeDuplicateMediaQueries - removes duplicated @media nested blocks;
  • mergeMediaQueries - merges non-adjacent @media at-rules by the same rules as mergeNonAdjacentBy* above;

How to use clean-css with build tools?

There is a number of 3rd party plugins to popular build tools:

How to use clean-css from web browser?



How to get started?

First clone the sources:

git clone

then install dependencies:

cd clean-css
npm install

then use any of the following commands to verify your copy:

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 lint JS sources with [JSHint](
npm test # to run all tests


Sorted alphabetically by GitHub handle:

  • @abarre (Anthony Barre) for improvements to @import processing;
  • @alexlamsl (Alex Lam S.L.) for testing early clean-css 4 versions, reporting bugs, and suggesting numerous improvements.
  • @altschuler (Simon Altschuler) for fixing @import processing inside comments;
  • @ben-eb (Ben Briggs) for sharing ideas about CSS optimizations;
  • @facelessuser (Isaac) for pointing out a flaw in clean-css' stateless mode;
  • @jmalonzo (Jan Michael Alonzo) for a patch removing node.js' old sys package;
  • @lukeapage (Luke Page) for suggestions and testing the source maps feature; Plus everyone else involved in #125 for pushing it forward;
  • @madwizard-thomas for sharing ideas about @import inlining and URL rebasing.
  • @ngyikp (Ng Yik Phang) for testing early clean-css 4 versions, reporting bugs, and suggesting numerous improvements.
  • @wagenet (Peter Wagenet) for suggesting improvements to @import inlining behavior;
  • @venemo (Timur Kristóf) for an outstanding contribution of advanced property optimizer for 2.2 release;
  • @vvo (Vincent Voyer) 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, like option to remove special comments and strip out URLs quotation, and pointing out numerous improvements like JSHint, media queries, etc.


clean-css is released under the MIT License.