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.

IMPORTANT: clean-css is now in a maintenance mode. PRs are still welcome, and I will try do an occasional bugfix release.

Table of Contents

• Node.js version support
• Install
• Use
  • What's new in version 5.3
  • What's new in version 5.0
  • What's new in version 4.2
  • What's new in version 4.1
  • Important: 4.0 breaking changes
  • Constructor options
  • Compatibility modes
  • Fetch option
  • Formatting options
  • Inlining options
  • Optimization levels
    • Level 0 optimizations
    • Level 1 optimizations
    • Level 2 optimizations
  • Plugins
  • Minify method
  • Promise interface
  • CLI utility
• FAQ
  • How to optimize multiple files?
  • How to process multiple files without concatenating them into one output file?
  • 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?
  • What errors and warnings are?
  • How to use clean-css with build tools?
  • How to use clean-css from web browser?
• Contributing
  • How to get started?
• Acknowledgments
• License

Node.js version support

clean-css requires Node.js 10.0+ (tested on Linux, OS X, and Windows)

Install

npm install --save-dev clean-css

Use

1var CleanCSS = require('clean-css');
2var input = 'a{font-weight:bold;}';
3var options = { /* options */ };
4var output = new CleanCSS(options).minify(input);

What's new in version 5.3

clean-css 5.3 introduces one new feature:

• variables can be optimized using level 1's variableValueOptimizers option, which accepts a list of value optimizers or a list of their names, e.g. variableValueOptimizers: ['color', 'fraction'].

What's new in version 5.0

clean-css 5.0 introduced some breaking changes:

• Node.js 6.x and 8.x are officially no longer supported;
• transform callback in level-1 optimizations is removed in favor of new plugins interface;
• changes default Internet Explorer compatibility from 10+ to >11, to revert the old default use { compatibility: 'ie10' } flag;
• changes default rebase option from true to false so URLs are not rebased by default. Please note that if you set rebaseTo option it still counts as setting rebase: true to preserve some of the backward compatibility.

And on the new features side of things:

• format options now accepts numerical values for all breaks, which will allow you to have more control over output formatting, e.g. format: {breaks: {afterComment: 2}} means clean-css will add two line breaks after each comment
• a new batch option (defaults to false) is added, when set to true it will process all inputs, given either as an array or a hash, without concatenating them.

What's new in version 4.2

clean-css 4.2 introduces 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;
• adds configurable line breaks via format: { breakWith: 'lf' } option.

What's new in version 4.1

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.

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. //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.

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;
• 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 false;
• 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:

1new CleanCSS({
2  compatibility: {
3    colors: {
4      hexAlpha: false, // controls 4- and 8-character hex color support
5      opacity: true // controls `rgba()` / `hsla()` color support
6    },
7    properties: {
8      backgroundClipMerging: true, // controls background-clip merging into shorthand
9      backgroundOriginMerging: true, // controls background-origin merging into shorthand
10      backgroundSizeMerging: true, // controls background-size merging into shorthand
11      colors: true, // controls color optimizations
12      ieBangHack: false, // controls keeping IE bang hack
13      ieFilters: false, // controls keeping IE `filter` / `-ms-filter`
14      iePrefixHack: false, // controls keeping IE prefix hack
15      ieSuffixHack: false, // controls keeping IE suffix hack
16      merging: true, // controls property merging based on understandability
17      shorterLengthUnits: false, // controls shortening pixel units into `pc`, `pt`, or `in` units
18      spaceAfterClosingBrace: true, // controls keeping space after closing brace - `url() no-repeat` into `url()no-repeat`
19      urlQuotes: true, // controls keeping quoting inside `url()`
20      zeroUnits: true // controls removal of units `0` value
21    },
22    selectors: {
23      adjacentSpace: false, // controls extra space before `nav` element
24      ie7Hack: true, // controls removal of IE7 selector hacks, e.g. `*+html...`
25      mergeablePseudoClasses: [':active', ...], // controls a whitelist of mergeable pseudo classes
26      mergeablePseudoElements: ['::after', ...], // controls a whitelist of mergeable pseudo elements
27      mergeLimit: 8191, // controls maximum number of selectors in a single rule (since 4.1.0)
28      multiplePseudoMerging: true // controls merging of rules with multiple pseudo classes / elements (since 4.1.0)
29    },
30    units: {
31      ch: true, // controls treating `ch` as a supported unit
32      in: true, // controls treating `in` as a supported unit
33      pc: true, // controls treating `pc` as a supported unit
34      pt: true, // controls treating `pt` as a supported unit
35      rem: true, // controls treating `rem` as a supported unit
36      vh: true, // controls treating `vh` as a supported unit
37      vm: true, // controls treating `vm` as a supported unit
38      vmax: true, // controls treating `vmax` as a supported unit
39      vmin: true // controls treating `vmin` as a supported unit
40    }
41  }
42})

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

1new CleanCSS({
2  compatibility: 'ie9,-properties.merging' // sets compatibility to IE9 mode with disabled property merging
3})

Fetch option

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

1var request = require('request');
2var source = '@import url(http://example.com/path/to/stylesheet.css);';
3new CleanCSS({
4  fetch: function (uri, inlineRequest, inlineTimeout, callback) {
5    request(uri, function (error, response, body) {
6      if (error) {
7        callback(error, null);
8      } else if (response && response.statusCode != 200) {
9        callback(response.statusCode, null);
10      } else {
11        callback(null, body);
12      }
13    });
14  }
15}).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.

Formatting options

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

1new CleanCSS({
2  format: 'beautify' // formats output in a really nice way
3})

and

1new CleanCSS({
2  format: 'keep-breaks' // formats output the default way but adds line breaks for improved readability
3})

however format option also accept a fine-grained set of options:

1new CleanCSS({
2  format: {
3    breaks: { // controls where to insert breaks
4      afterAtRule: false, // controls if a line break comes after an at-rule; e.g. `@charset`; defaults to `false`
5      afterBlockBegins: false, // controls if a line break comes after a block begins; e.g. `@media`; defaults to `false`
6      afterBlockEnds: false, // controls if a line break comes after a block ends, defaults to `false`
7      afterComment: false, // controls if a line break comes after a comment; defaults to `false`
8      afterProperty: false, // controls if a line break comes after a property; defaults to `false`
9      afterRuleBegins: false, // controls if a line break comes after a rule begins; defaults to `false`
10      afterRuleEnds: false, // controls if a line break comes after a rule ends; defaults to `false`
11      beforeBlockEnds: false, // controls if a line break comes before a block ends; defaults to `false`
12      betweenSelectors: false // controls if a line break comes between selectors; defaults to `false`
13    },
14    breakWith: '\n', // controls the new line character, can be `'\r\n'` or `'\n'` (aliased as `'windows'` and `'unix'` or `'crlf'` and `'lf'`); defaults to system one, so former on Windows and latter on Unix
15    indentBy: 0, // controls number of characters to indent with; defaults to `0`
16    indentWith: 'space', // controls a character to indent with, can be `'space'` or `'tab'`; defaults to `'space'`
17    spaces: { // controls where to insert spaces
18      aroundSelectorRelation: false, // controls if spaces come around selector relations; e.g. `div > a`; defaults to `false`
19      beforeBlockBegins: false, // controls if a space comes before a block begins; e.g. `.block {`; defaults to `false`
20      beforeValue: false // controls if a space comes before a value; e.g. `width: 1rem`; defaults to `false`
21    },
22    wrapAt: false, // controls maximum line length; defaults to `false`
23    semicolonAfterLastProperty: false // controls removing trailing semicolons in rule; defaults to `false` - means remove
24  }
25})

Also since clean-css 5.0 you can use numerical values for all line breaks, which will repeat a line break that many times, e.g:

1  new CleanCSS({
2    format: {
3      breaks: {
4        afterAtRule: 2,
5        afterBlockBegins: 1, // 1 is synonymous with `true`
6        afterBlockEnds: 2,
7        afterComment: 1,
8        afterProperty: 1,
9        afterRuleBegins: 1,
10        afterRuleEnds: 1,
11        beforeBlockEnds: 1,
12        betweenSelectors: 0 // 0 is synonymous with `false`
13      }
14    }
15  })

which will add nicer spacing between at rules and blocks.

Inlining options

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

1new CleanCSS({
2  inline: ['local'] // default; enables local inlining only
3})

1new CleanCSS({
2  inline: ['none'] // disables all inlining
3})

1// introduced in clean-css 4.1.0
2
3new CleanCSS({
4  inline: false // disables all inlining (alias to `['none']`)
5})

1new CleanCSS({
2  inline: ['all'] // enables all inlining, same as ['local', 'remote']
3})

1new CleanCSS({
2  inline: ['local', 'mydomain.example.com'] // enables local inlining plus given remote source
3})

1new CleanCSS({
2  inline: ['local', 'remote', '!fonts.googleapis.com'] // enables all inlining but from given remote source
3})

Optimization levels

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

1new CleanCSS({
2  level: 2
3})

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:

1new CleanCSS({
2  level: {
3    1: {
4      cleanupCharsets: true, // controls `@charset` moving to the front of a stylesheet; defaults to `true`
5      normalizeUrls: true, // controls URL normalization; defaults to `true`
6      optimizeBackground: true, // controls `background` property optimizations; defaults to `true`
7      optimizeBorderRadius: true, // controls `border-radius` property optimizations; defaults to `true`
8      optimizeFilter: true, // controls `filter` property optimizations; defaults to `true`
9      optimizeFont: true, // controls `font` property optimizations; defaults to `true`
10      optimizeFontWeight: true, // controls `font-weight` property optimizations; defaults to `true`
11      optimizeOutline: true, // controls `outline` property optimizations; defaults to `true`
12      removeEmpty: true, // controls removing empty rules and nested blocks; defaults to `true`
13      removeNegativePaddings: true, // controls removing negative paddings; defaults to `true`
14      removeQuotes: true, // controls removing quotes when unnecessary; defaults to `true`
15      removeWhitespace: true, // controls removing unused whitespace; defaults to `true`
16      replaceMultipleZeros: true, // controls removing redundant zeros; defaults to `true`
17      replaceTimeUnits: true, // controls replacing time units with shorter values; defaults to `true`
18      replaceZeroUnits: true, // controls replacing zero values with units; defaults to `true`
19      roundingPrecision: false, // rounds pixel values to `N` decimal places; `false` disables rounding; defaults to `false`
20      selectorsSortingMethod: 'standard', // denotes selector sorting method; can be `'natural'` or `'standard'`, `'none'`, or false (the last two since 4.1.0); defaults to `'standard'`
21      specialComments: 'all', // denotes a number of /*! ... */ comments preserved; defaults to `all`
22      tidyAtRules: true, // controls at-rules (e.g. `@charset`, `@import`) optimizing; defaults to `true`
23      tidyBlockScopes: true, // controls block scopes (e.g. `@media`) optimizing; defaults to `true`
24      tidySelectors: true, // controls selectors optimizing; defaults to `true`,
25      variableValueOptimizers: [] // controls value optimizers which are applied to variables
26    }
27  }
28});

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

1new CleanCSS({
2  level: {
3    1: {
4      all: false, // set all values to `false`
5      tidySelectors: true // turns on optimizing selectors
6    }
7  }
8});

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:

1new CleanCSS({
2  level: {
3    2: {
4      mergeAdjacentRules: true, // controls adjacent rules merging; defaults to true
5      mergeIntoShorthands: true, // controls merging properties into shorthands; defaults to true
6      mergeMedia: true, // controls `@media` merging; defaults to true
7      mergeNonAdjacentRules: true, // controls non-adjacent rule merging; defaults to true
8      mergeSemantically: false, // controls semantic merging; defaults to false
9      overrideProperties: true, // controls property overriding based on understandability; defaults to true
10      removeEmpty: true, // controls removing empty rules and nested blocks; defaults to `true`
11      reduceNonAdjacentRules: true, // controls non-adjacent rule reducing; defaults to true
12      removeDuplicateFontRules: true, // controls duplicate `@font-face` removing; defaults to true
13      removeDuplicateMediaBlocks: true, // controls duplicate `@media` removing; defaults to true
14      removeDuplicateRules: true, // controls duplicate rules removing; defaults to true
15      removeUnusedAtRules: false, // controls unused at rule removing; defaults to false (available since 4.1.0)
16      restructureRules: false, // controls rule restructuring; defaults to false
17      skipProperties: [] // controls which properties won't be optimized, defaults to `[]` which means all will be optimized (since 4.1.0)
18    }
19  }
20});

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

1new CleanCSS({
2  level: {
3    2: {
4      all: false, // sets all values to `false`
5      removeDuplicateRules: true // turns on removing duplicate rules
6    }
7  }
8});

Plugins

In clean-css version 5 and above you can define plugins which run alongside level 1 and level 2 optimizations, e.g.

1var myPlugin = {
2  level1: {
3    property: function removeRepeatedBackgroundRepeat(_rule, property, _options) {
4      // So `background-repeat:no-repeat no-repeat` becomes `background-repeat:no-repeat`
5      if (property.name == 'background-repeat' && property.value.length == 2 && property.value[0][1] == property.value[1][1]) {
6        property.value.pop();
7        property.dirty = true;
8      }
9    }
10  }
11}
12
13new CleanCSS({plugins: [myPlugin]})
14

Search test\module-test.js for plugins or check out lib/optimizer/level-1/property-optimizers and lib/optimizer/level-1/value-optimizers for more examples.

Important: To rewrite your old transform as a plugin, check out this commit.

Minify method

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

1var output = new CleanCSS(options).minify(source);

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

1console.log(output.styles); // optimized output CSS as a string
2console.log(output.sourceMap); // output source map if requested with `sourceMap` option
3console.log(output.errors); // a list of errors raised
4console.log(output.warnings); // a list of warnings raised
5console.log(output.stats.originalSize); // original content size after import inlining
6console.log(output.stats.minifiedSize); // optimized content size
7console.log(output.stats.timeSpent); // time spent on optimizations in milliseconds
8console.log(output.stats.efficiency); // `(originalSize - minifiedSize) / originalSize`, e.g. 0.25 if size is reduced from 100 bytes to 75 bytes

Example: Minifying a CSS string:

1const CleanCSS = require("clean-css");
2
3const output = new CleanCSS().minify(`
4
5  a {
6    color: blue;
7  }
8  div {
9    margin: 5px
10  }
11
12`);
13
14console.log(output);
15
16// Log:
17{
18  styles: 'a{color:#00f}div{margin:5px}',
19  stats: {
20    efficiency: 0.6704545454545454,
21    minifiedSize: 29,
22    originalSize: 88,
23    timeSpent: 6
24  },
25  errors: [],
26  inlinedStylesheets: [],
27  warnings: []
28}

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

1var output = new CleanCSS(options).minify(source, inputSourceMap);

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

1new CleanCSS(options).minify(source, function (error, output) {
2  // `output` is the same as in the synchronous call above
3});

To optimize a single file, without reading it first, pass a path to it to minify method as follows:

1var output = new CleanCSS(options).minify(['path/to/file.css'])

(if you won't enclose the path in an array, it will be treated as a CSS source instead).

There are several ways to optimize multiple files at the same time, see How to optimize multiple files?.

Promise interface

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

1new CleanCSS({ returnPromise: true })
2  .minify(source)
3  .then(function (output) { console.log(output.styles); })
4  .catch(function (error) { // deal with errors });

CLI utility

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/clean-css/clean-css-cli.

FAQ

How to optimize multiple files?

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

1new CleanCSS().minify(['path/to/file/one', 'path/to/file/two']);

1new CleanCSS().minify({
2  'path/to/file/one': {
3    styles: 'contents of file one'
4  },
5  'path/to/file/two': {
6    styles: 'contents of file two'
7  }
8});

1new CleanCSS().minify([
2  {'path/to/file/one': {styles: 'contents of file one'}},
3  {'path/to/file/two': {styles: 'contents of file two'}}
4]);

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.

How to process multiple files without concatenating them into one output file?

Since clean-css 5.0 you can, when passing an array of paths, hash, or array of hashes (see above), ask clean-css not to join styles into one output, but instead return stylesheets optimized one by one, e.g.

1var output = new CleanCSS({ batch: true }).minify(['path/to/file/one', 'path/to/file/two']);
2var outputOfFile1 = output['path/to/file/one'].styles // all other fields, like errors, warnings, or stats are there too
3var outputOfFile2 = output['path/to/file/two'].styles

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.:

1var source = '@import url(http://example.com/path/to/remote/styles);';
2new CleanCSS({ inline: ['remote'] }).minify(source, function (error, output) {
3  // output.styles
4});

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

How to apply arbitrary transformations to CSS properties?

Please see plugins.

How to specify a custom rounding precision?

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

1new CleanCSS({
2  level: {
3    1: {
4      roundingPrecision: 'all=3,px=5'
5    }
6  }
7}).minify(source)

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

How to optimize a stylesheet with custom rpx units?

Since rpx is a non standard unit (see #1074), it will be dropped by default as an invalid value.

However you can treat rpx units as regular ones:

1new CleanCSS({
2  compatibility: {
3    customUnits: {
4      rpx: true
5    }
6  }
7}).minify(source)

How to keep a CSS fragment intact?

Note: available since 4.2.0.

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

1.block-1 {
2  color: red
3}
4/* clean-css ignore:start */
5.block-special {
6  color: transparent
7}
8/* clean-css ignore:end */
9.block-2 {
10  margin: 0
11}

Optimizing this CSS will result in the following output:

1.block-1{color:red}
2.block-special {
3  color: transparent
4}
5.block-2{margin:0}

How to preserve a comment block?

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

1/*!
2  Important comments included in optimized output.
3*/

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.:

1new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory })
2  .minify(source, function (error, output) {
3    // access output.sourceMap for SourceMapGenerator object
4    // see https://github.com/mozilla/source-map/#sourcemapgenerator for more details
5});

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

1new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory })
2  .minify(source, inputSourceMap, function (error, output) {
3    // access output.sourceMap to access SourceMapGenerator object
4    // see https://github.com/mozilla/source-map/#sourcemapgenerator for more details
5});

or even multiple input source maps at once:

1new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory }).minify({
2  'path/to/source/1': {
3    styles: '...styles...',
4    sourceMap: '...source-map...'
5  },
6  'path/to/source/2': {
7    styles: '...styles...',
8    sourceMap: '...source-map...'
9  }
10}, function (error, output) {
11  // access output.sourceMap as above
12});

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

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

1new CleanCSS({
2  level: {
3    1: {
4      all: true,
5      normalizeUrls: false
6    },
7    2: {
8      restructureRules: true
9    }
10  }
11})

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;

What errors and warnings are?

If clean-css encounters invalid CSS, it will try to remove the invalid part and continue optimizing the rest of the code. It will make you aware of the problem by generating an error or warning. Although clean-css can work with invalid CSS, it is always recommended that you fix warnings and errors in your CSS.

Example: Minify invalid CSS, resulting in two warnings:

1const CleanCSS = require("clean-css");
2
3const output = new CleanCSS().minify(`
4
5  a {
6    -notarealproperty-: 5px;
7    color:
8  }
9  div {
10    margin: 5px
11  }
12
13`);
14
15console.log(output);
16
17// Log:
18{
19  styles: 'div{margin:5px}',
20  stats: {
21    efficiency: 0.8695652173913043,
22    minifiedSize: 15,
23    originalSize: 115,
24    timeSpent: 1
25  },
26  errors: [],
27  inlinedStylesheets: [],
28  warnings: [
29    "Invalid property name '-notarealproperty-' at 4:8. Ignoring.",
30    "Empty property 'color' at 5:8. Ignoring."
31  ]
32}

Example: Minify invalid CSS, resulting in one error:

1const CleanCSS = require("clean-css");
2
3const output = new CleanCSS().minify(`
4
5  @import "idontexist.css";
6  a {
7    color: blue;
8  }
9  div {
10    margin: 5px
11  }
12
13`);
14
15console.log(output);
16
17// Log:
18{
19  styles: 'a{color:#00f}div{margin:5px}',
20  stats: {
21    efficiency: 0.7627118644067796,
22    minifiedSize: 28,
23    originalSize: 118,
24    timeSpent: 2
25  },
26  errors: [
27    'Ignoring local @import of "idontexist.css" as resource is missing.'
28  ],
29  inlinedStylesheets: [],
30  warnings: []
31}

Clean-css for Gulp

An example of how you can include clean-css in gulp

1const { src, dest, series } = require('gulp');
2const CleanCSS = require('clean-css');
3const concat = require('gulp-concat');
4
5function css() {
6    const options = {
7        compatibility: '*', // (default) - Internet Explorer 10+ compatibility mode
8        inline: ['all'], // enables all inlining, same as ['local', 'remote']
9        level: 2 // Optimization levels. The level option can be either 0, 1 (default), or 2, e.g.
10        // Please note that level 1 optimization options are generally safe while level 2 optimizations should be safe for most users.
11    };
12
13    return src('app/**/*.css')
14        .pipe(concat('style.min.css'))
15        .on('data', function(file) {
16            const bufferFile = new CleanCSS(options).minify(file.contents)
17            return file.contents = Buffer.from(bufferFile.styles)
18        })
19        .pipe(dest('build'))
20}
21exports.css = series(css)

How to use clean-css with build tools?

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

How to use clean-css from web browser?

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

Contributing

See CONTRIBUTING.md.

How to get started?

First clone the sources:

1git clone git@github.com:clean-css/clean-css.git

then install dependencies:

1cd clean-css
2npm install

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

1npm run bench # for clean-css benchmarks (see [test/bench.js](https://github.com/clean-css/clean-css/blob/master/test/bench.js) for details)
2npm run browserify # to create the browser-ready clean-css version
3npm run check # to lint JS sources with [JSHint](https://github.com/jshint/jshint/)
4npm test # to run all tests

Acknowledgments

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;
• @davisjam (Jamie Davis) for disclosing ReDOS vulnerabilities;
• @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.

License

clean-css is released under the MIT License.