clean-css
Advanced tools
clean-css is a fast and efficient CSS optimizer for Node.js and the web. It minifies CSS by removing whitespace, comments, and unnecessary characters, merging identical rules, and applying other optimization techniques. It can be used to reduce the size of CSS files, which can lead to faster page load times and improved performance.
Minification
This feature allows you to minify CSS by removing unnecessary characters and whitespace, which results in smaller file sizes.
const CleanCSS = require('clean-css');
new CleanCSS().minify('a { font-weight: bold; }', function (error, output) {
console.log(output.styles);
});Source Map Generation
clean-css can generate source maps for the minified CSS, which is useful for debugging minified code in the browser.
const CleanCSS = require('clean-css');
new CleanCSS({ sourceMap: true, sourceMapInlineSources: true }).minify({ 'styles.css': { styles: 'a { color: #000 }' } }, function (error, output) {
console.log(output.sourceMap.toString());
});Merging of Identical CSS Rules
This feature merges identical CSS rules into one to reduce file size and redundancy in the code.
const CleanCSS = require('clean-css');
new CleanCSS().minify('a { color: red; } a { color: red; }', function (error, output) {
console.log(output.styles);
});Optimizing CSS Properties
clean-css can optimize CSS properties, such as converting hex color codes to shorter names when possible.
const CleanCSS = require('clean-css');
new CleanCSS().minify('a { color: #FF0000; }', function (error, output) {
console.log(output.styles);
});cssnano is a modular CSS minifier that utilizes PostCSS. It offers optimizations like reducing calc() references, z-index rebasing, and discarding unused at-rules. Compared to clean-css, cssnano provides a plugin system and can be more configurable due to its modular nature.
uglifycss is a simple CSS minifier that works both in Node.js and the browser. It is less feature-rich compared to clean-css and does not support source maps, but it is straightforward to use for basic minification tasks.
csso (CSS Optimizer) is a CSS minifier with structural optimizations. It can restructure CSS rules to reduce file size and also merge selectors and properties. csso is known for its aggressive optimization techniques, which sometimes can be more efficient than clean-css, but may also require careful testing to ensure the final CSS behaves as expected.
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
@imports correctly?clean-css requires Node.js 4.0+ (tested on Linux, OS X, and Windows)
npm install --save-dev 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:
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;inliner: { request: ..., timeout: ... } option into inlineRequest and inlineTimeout options;//fonts.googleapis.com/css?family=Domine:700, are not inlined anymore;{ compatibility: 'ie9' } flag;keepSpecialComments to specialComments;roundingPrecision and specialComments to level 1 optimizations options, see examples;mediaMerging, restructuring, semanticMerging, and shorthandCompacting to level 2 optimizations options, see examples below;shorthandCompacting option to mergeIntoShorthands;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;font shorthand and font-* longhand optimizers;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;animation shorthand and animation-* longhand optimizers;removeUnusedAtRules level 2 optimization controlling removal of unused @counter-style, @font-face, @keyframes, and @namespace at rules;clean-css 4.2 introduces the following changes / features:
process method for compatibility with optimize-css-assets-webpack-plugin;transition property optimizer;/* clean-css ignore:start */ and /* clean-css ignore:end */ comments;transform callback, see example;format: { breakWith: 'lf' } option.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 modenew CleanCSS({ compatibility: 'ie9' }) - Internet Explorer 9+ compatibility modenew CleanCSS({ compatibility: 'ie8' }) - Internet Explorer 8+ compatibility modenew CleanCSS({ compatibility: 'ie7' }) - Internet Explorer 7+ compatibility modeEach 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`
},
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
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`,
semicolonAfterLastProperty: false, // controls removing trailing semicolons in rule; defaults to `false` - means remove
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); // `(originalSize - minifiedSize) / originalSize`, e.g. 0.25 if size is 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.
@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(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:
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:
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:
@import processing;@import processing inside comments;minify method source traversal in ES6;sys package;@import inlining and URL rebasing.@import inlining behavior;clean-css is released under the MIT License.
FAQs
A well-tested CSS minifier
We found that clean-css demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 2 open source maintainers collaborating on the project.
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