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
• Install
• Use
  • Important: 4.0 breaking changes
  • What's new in version 4.1
  • What's new in version 4.2
  • Constructor options
  • Compatibility modes
  • Fetch option
  • Formatting options
  • Inlining options
  • Optimization levels
    • Level 0 optimizations
    • Level 1 optimizations
    • Level 2 optimizations
  • Minify method
  • Promise interface
  • CLI utility
• FAQ
  • How to optimize multiple files?
  • How to process remote @imports correctly?
  • How to apply arbitrary transformations to CSS properties?
  • How to specify a custom rounding precision?
  • How to keep a CSS fragment intact?
  • How to preserve a comment block?
  • How to rebase relative image URLs?
  • How to work with source maps?
  • How to apply level 1 & 2 optimizations at the same time?
  • What level 2 optimizations do?
  • How to use clean-css with build tools?
  • How to use clean-css from web browser?
• Contributing
  • How to get started?
• Acknowledgments
• License

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);

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. //fonts.googleapis.com/css?family=Domine:700, 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.

clean-css 4.1 introduces the following changes / features:

• inline: false as an alias to inline: ['none'];
• multiplePseudoMerging compatibility flag controlling merging of rules with multiple pseudo classes / elements;
• removeEmpty flag in level 1 optimizations controlling removal of rules and nested blocks;
• removeEmpty flag in level 2 optimizations controlling removal of rules and nested blocks;
• compatibility: { selectors: { mergeLimit: <number> } } flag in compatibility settings controlling maximum number of selectors in a single rule;
• minify method improved signature accepting a list of hashes for a predictable traversal;
• selectorsSortingMethod level 1 optimization allows false or 'none' for disabling selector sorting;
• fetch option controlling a function for handling remote requests;
• new font shorthand and font-* longhand optimizers;
• removal of optimizeFont flag in level 1 optimizations due to new font shorthand optimizer;
• skipProperties flag in level 2 optimizations controlling which properties won't be optimized;
• new animation shorthand and animation-* longhand optimizers;
• removeUnusedAtRules level 2 optimization controlling removal of unused @counter-style, @font-face, @keyframes, and @namespace at rules;
• the web interface gets an improved settings panel with "reset to defaults", instant option changes, and settings being persisted across sessions.

clean-css 4.2 will introduce the following changes / features:

• Adds process method for compatibility with optimize-css-assets-webpack-plugin;
• new transition property optimizer;
• preserves any CSS content between /* clean-css ignore:start */ and /* clean-css ignore:end */ comments;
• allows filtering based on selector in transform callback, see example;

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;
• fetch - controls a function for handling remote requests; see fetch option for examples (since 4.1.0);
• 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.

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 mergeLimit: 8191, // controls maximum number of selectors in a single rule (since 4.1.0) multiplePseudoMerging: true // controls merging of rules with multiple pseudo classes / elements (since 4.1.0) }, 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 })

The fetch option accepts a function which handles remote resource fetching, e.g.

var request = require('request'); var source = '@import url(http://example.com/path/to/stylesheet.css);'; new CleanCSS({ fetch: function (uri, inlineRequest, inlineTimeout, callback) { request(uri, function (error, response, body) { if (error) { callback(error, null); } else if (response && response.statusCode != 200) { callback(response.statusCode, null); } else { callback(null, body); } }); } }).minify(source);

This option provides a convenient way of overriding the default fetching logic if it doesn't support a particular feature, say CONNECT proxies.

Unless given, the default loadRemoteResource logic is used.

By default output CSS is formatted without any whitespace unless a format option is given. First of all there are two shorthands:

new CleanCSS({ format: 'beautify' // formats output in a really nice way })

and

new CleanCSS({ format: 'keep-breaks' // formats output the default way but adds line breaks for improved readability })

however format option also accept a fine-grained set of 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` } })

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

new CleanCSS({ inline: ['local'] // default; enables local inlining only })

new CleanCSS({ inline: ['none'] // disables all inlining })

// introduced in clean-css 4.1.0 new CleanCSS({ inline: false // disables all inlining (alias to `['none']`) })

new CleanCSS({ inline: ['all'] // enables all inlining, same as ['local', 'remote'] })

new CleanCSS({ inline: ['local', 'mydomain.example.com'] // enables local inlining plus given remote source })

new CleanCSS({ inline: ['local', 'remote', '!fonts.googleapis.com'] // enables all inlining but from given remote source })

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

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 optimizations; defaults to `true` optimizeBorderRadius: true, // controls `border-radius` property optimizations; defaults to `true` optimizeFilter: true, // controls `filter` property optimizations; defaults to `true` optimizeFont: true, // controls `font` property optimizations; defaults to `true` optimizeFontWeight: true, // controls `font-weight` property optimizations; defaults to `true` optimizeOutline: true, // controls `outline` property optimizations; defaults to `true` removeEmpty: true, // controls removing empty rules and nested blocks; 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'`, `'none'`, or false (the last two since 4.1.0); 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 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 removeEmpty: true, // controls removing empty rules and nested blocks; 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 removeUnusedAtRules: false, // controls unused at rule removing; defaults to false (available since 4.1.0) restructureRules: false, // controls rule restructuring; defaults to false skipProperties: [] // controls which properties won't be optimized, defaults to `[]` which means all will be optimized (since 4.1.0) } } });

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 } } });

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 });

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

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

Clean-css has an associated command line utility that can be installed separately using npm install clean-css-cli. For more detailed information, please visit https://github.com/jakubpawlowicz/clean-css-cli.

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

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' } });

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

Passing an array of hashes allows you to explicitly specify the order in which the input files are concatenated. Whereas when you use a single hash the order is determined by the traversal order of object properties - available since 4.1.0.

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

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(http://example.com/path/to/remote/styles);'; 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.

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, selector /* `selector` available since 4.2.0-pre */) { if (propertyName == 'background-image' && propertyValue.indexOf('/path/to') > -1) { return propertyValue.replace('/path/to', '../valid/path/to'); } } } } }).minify(source); console.log(output.styles); # => .block{background-image:url(../valid/path/to/image.png)}

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

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' } } }).minify(source)

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

Note: available in the current master, to be released in 4.2.0.

Wrap the CSS fragment in special comments which instruct clean-css to preserve it, e.g.

.block-1 { color: red } /* clean-css ignore:start */ .block-special { color: transparent } /* clean-css ignore:end */ .block-2 { margin: 0 }

Optimizing this CSS will result in the following output:

.block-1{color:red} .block-special { color: transparent } .block-2{margin:0}

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

/*!  Important comments included in optimized output. */

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.

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 https://github.com/mozilla/source-map/#sourcemapgenerator 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 https://github.com/mozilla/source-map/#sourcemapgenerator 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 });

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.

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;

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

• Broccoli: broccoli-clean-css
• Brunch: clean-css-brunch
• Grunt: grunt-contrib-cssmin
• Gulp: gulp-clean-css
• Gulp: using vinyl-map as a wrapper - courtesy of @sogko
• component-builder2: builder-clean-css
• Metalsmith: metalsmith-clean-css
• Lasso: lasso-clean-css
• Start: start-clean-css

• https://jakubpawlowicz.github.io/clean-css/ (official web interface)
• http://refresh-sf.com/
• http://adamburgess.github.io/clean-css-online/

See CONTRIBUTING.md.

First clone the sources:

git clone git@github.com:jakubpawlowicz/clean-css.git

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](https://github.com/jakubpawlowicz/clean-css/blob/master/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](https://github.com/jshint/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;
• @grandrath (Martin Grandrath) for improving minify method source traversal in ES6;
• @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.