clean-css vs cssnano vs gulp-clean-css vs postcss-clean vs postcss-minify

CSS Minification Strategies: Standalone Engines vs. PostCSS Ecosystem

This comparison evaluates five distinct approaches to CSS minification in modern frontend workflows. clean-css acts as a powerful, standalone optimization engine with advanced logic for restructuring stylesheets. cssnano serves as the de facto standard minifier within the PostCSS ecosystem, offering a plugin-based architecture that integrates seamlessly with tools like Webpack and Vite. gulp-clean-css is a legacy Gulp wrapper around the clean-css engine, while postcss-clean and postcss-minify represent older or alternative attempts to bridge clean-css or basic minification into PostCSS pipelines. The analysis focuses on architectural fit, integration patterns, and current maintenance status to guide developers in selecting the right tool for their build system.

Npm Package Weekly Downloads Trend

3 Years

Github Stars Ranking

Stat Detail

Package	Downloads	Stars	Size	Issues	Publish	License
Package	Downloads	Stars	Size	Issues	Publish	License
clean-css	0	4,197	493 kB	46	3 years ago	MIT
cssnano	0	-	8.01 kB	-	3 days ago	MIT
gulp-clean-css	0	642	-	14	6 years ago	MIT
postcss-clean	0	43	-	12	6 years ago	MIT
postcss-minify	0	7	8.22 kB	0	a year ago	MIT

CSS Minification Deep Dive: clean-css vs. cssnano vs. Legacy Wrappers

Minifying CSS is a critical step in any frontend build pipeline. It reduces file size, improves load times, and lowers bandwidth costs. However, not all minifiers are created equal. Some are standalone powerhouses, while others are specialized plugins designed to fit into broader ecosystems like PostCSS. This analysis breaks down the technical realities of clean-css, cssnano, and their various wrappers to help you make an informed architectural decision.

🏗️ Architecture: Standalone Engine vs. PostCSS Plugin

The most fundamental difference lies in how these tools are designed to run. clean-css is a standalone library. It does not depend on PostCSS. You can run it in a Node.js script, a CLI command, or wrap it in any build tool you like. In contrast, cssnano is built specifically as a PostCSS plugin. It relies on the PostCSS AST (Abstract Syntax Tree) parser and processor.

clean-css operates directly on CSS strings or source maps. It has its own parsing logic optimized for speed and deep restructuring.

// clean-css: Standalone usage
const CleanCSS = require('clean-css');
const source = 'body { color: red; }';

const output = new CleanCSS({ level: 2 }).minify(source);
console.log(output.styles); // "body{color:red}"

cssnano must be used as part of a PostCSS processor chain. It transforms the AST generated by PostCSS.

// cssnano: PostCSS plugin usage
const postcss = require('postcss');
const cssnano = require('cssnano');

const result = await postcss([cssnano])
  .process('body { color: red; }', { from: undefined });

console.log(result.css); // "body{color:red}"

gulp-clean-css is a Gulp-specific wrapper. It takes the standalone clean-css engine and adapts it to work with Gulp's vinyl file stream system.

// gulp-clean-css: Gulp stream usage
const cleanCss = require('gulp-clean-css');
const gulp = require('gulp');

gulp.src('src/*.css')
  .pipe(cleanCss({ compatibility: 'ie8' }))
  .pipe(gulp.dest('dist'));

postcss-clean and postcss-minify attempt to bring minification into PostCSS but lack the robust ecosystem integration of cssnano. They often act as thin shims.

// postcss-clean: Alternative PostCSS wrapper
const postcss = require('postcss');
const clean = require('postcss-clean');

// Usage is similar to cssnano but backed by clean-css internally
const result = await postcss([clean()])
  .process('body { color: red; }', { from: undefined });

⚙️ Optimization Levels: Deep Restructuring vs. Safe Trimming

Not all minification is just about removing whitespace. Advanced tools reorder rules, merge selectors, and remove duplicate properties to save even more bytes. clean-css is famous for its "Level 2" optimizations, which perform aggressive restructuring.

clean-css allows you to toggle optimization levels. Level 1 is safe (whitespace removal), while Level 2 restructures the CSS.

// clean-css: Aggressive Level 2 optimization
const CleanCSS = require('clean-css');

// Merges duplicate selectors and reorders properties
const output = new CleanCSS({ level: 2 }).minify(`
  .btn { color: blue; }
  .btn { background: white; }
`);
// Output: ".btn{background:white;color:blue}"

cssnano achieves similar results through its preset system. The default preset includes aggressive optimizations, but you can customize them.

// cssnano: Configurable presets
const cssnano = require('cssnano');

// Customize specific plugins within cssnano
const config = {
  preset: ['default', { discardComments: { removeAll: true } }]
};

const result = await postcss([cssnano(config)])
  .process('.btn { color: blue; } .btn { background: white; }', { from: undefined });

gulp-clean-css passes options directly to the underlying clean-css engine, so it supports the same levels, but you are locked into the Gulp stream pattern.

// gulp-clean-css: Passing level options
const cleanCss = require('gulp-clean-css');

// Enables level 2 optimizations within the Gulp pipe
.pipe(cleanCss({ level: 2 }));

postcss-clean exposes clean-css options but often suffers from version lag, meaning you might not get the latest Level 2 improvements immediately.

// postcss-clean: Option passing
const clean = require('postcss-clean');

// Attempts to pass level options, but API stability varies
postcss([clean({ level: 2 })]).process(css, { from: undefined });

postcss-minify typically offers only basic minification (Level 1 equivalent). It lacks the deep restructuring capabilities of the others.

// postcss-minify: Basic minification only
const minify = require('postcss-minify');

// Only removes whitespace and comments; no rule merging
postcss([minify()]).process(css, { from: undefined });

🔄 Ecosystem Integration: Webpack, Vite, and Gulp

Your choice often depends on your build tool. Modern tools like Webpack and Vite rely heavily on PostCSS loaders.

cssnano is the native choice for Webpack and Vite. It plugs directly into postcss-loader.

// webpack.config.js: Using cssnano
module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          'style-loader',
          'css-loader',
          {
            loader: 'postcss-loader',
            options: {
              postcssOptions: {
                plugins: [cssnano({ preset: 'default' })]
              }
            }
          }
        ]
      }
    ]
  }
};

clean-css requires a specific loader (like clean-css-loader) or a custom plugin in Webpack, adding extra configuration overhead.

// webpack.config.js: Using clean-css via loader
const CleanCssPlugin = require('clean-css-webpack-plugin');

module.exports = {
  plugins: [
    new CleanCssPlugin({
      test: /\.css$/g,
      options: { level: 2 }
    })
  ]
};

gulp-clean-css is strictly for Gulp users. If you are migrating away from Gulp, this package becomes a liability.

// gulpfile.js: Legacy Gulp setup
const gulp = require('gulp');
const cleanCss = require('gulp-clean-css');

gulp.task('minify-css', () => {
  return gulp.src('src/**/*.css')
    .pipe(cleanCss())
    .pipe(gulp.dest('dist'));
});

postcss-clean and postcss-minify can technically run in Webpack/Vite via postcss-loader, but they offer no advantage over cssnano and introduce maintenance risks.

// postcss.config.js: Using alternative plugins
module.exports = {
  plugins: [
    require('postcss-clean')(), // Works, but why not cssnano?
    require('autoprefixer')
  ]
};

⚠️ Maintenance Status and Deprecation Warnings

A critical factor in architectural decisions is the long-term viability of the package.

cssnano: Actively maintained. It is the community standard. Safe for production.
clean-css: Actively maintained. Excellent for standalone needs. Safe for production.
gulp-clean-css: Maintenance mode. It works, but updates depend on the wrapper author, not the core engine team. Avoid for new greenfield projects.
postcss-clean: Low activity. The repository sees infrequent updates. It is redundant given cssnano.
postcss-minify: Deprecated / Unmaintained. This package should NOT be used in new projects. It lacks the features and security updates of modern alternatives.

// ❌ DO NOT USE: postcss-minify in new projects
// Risk: No security patches, missing modern CSS features
const minify = require('postcss-minify'); 

// ✅ USE: cssnano for PostCSS workflows
const cssnano = require('cssnano');

📊 Summary: Feature Matrix

Feature	`clean-css`	`cssnano`	`gulp-clean-css`	`postcss-clean`	`postcss-minify`
Type	Standalone Engine	PostCSS Plugin	Gulp Wrapper	PostCSS Wrapper	PostCSS Plugin
Optimization	Level 1 & 2 (Advanced)	Preset-based (Advanced)	Level 1 & 2	Level 1 & 2	Basic Only
Ecosystem	Any (Node, CLI)	PostCSS (Webpack/Vite)	Gulp Only	PostCSS	PostCSS
Maintenance	✅ Active	✅ Active	⚠️ Legacy	⚠️ Low Activity	❌ Deprecated
Recommendation	High (Standalone)	High (PostCSS)	Low (Legacy)	Low (Redundant)	None (Avoid)

💡 Final Architectural Recommendation

For most modern frontend teams, cssnano is the correct choice. It integrates natively with the PostCSS ecosystem used by Webpack, Vite, and Rollup, providing a balance of safety, performance, and configurability. Its plugin architecture ensures you only pay for the optimizations you need.

Choose clean-css only if you are building a custom tooling pipeline that does not use PostCSS, or if you require its specific Level 2 restructuring algorithms in a standalone Node.js script.

Avoid gulp-clean-css, postcss-clean, and postcss-minify. The first locks you into a legacy build system, while the latter two are redundant or unmaintained wrappers that add unnecessary risk to your supply chain. Stick to the maintained standards to ensure your build pipeline remains robust and secure.

How to Choose: clean-css vs cssnano vs gulp-clean-css vs postcss-clean vs postcss-minify

clean-css:
Choose clean-css if you need a standalone, high-performance minifier with advanced optimization features (like level 2 restructuring) that operates independently of PostCSS. It is ideal for custom Node.js scripts, non-PostCSS build pipelines, or scenarios where you require fine-grained control over optimization levels without pulling in the entire PostCSS ecosystem.
cssnano:
Choose cssnano if you are already using PostCSS in your workflow (e.g., with Webpack, Vite, or Rollup). It is the industry standard for PostCSS-based minification, offering a modular plugin system that allows you to toggle specific optimizations (like merging rules or discarding comments) and ensuring consistent output across the entire CSS processing chain.
gulp-clean-css:
Avoid choosing gulp-clean-css for new projects. It is a wrapper specifically for Gulp 3/4 pipelines and often lags behind the latest features of the underlying clean-css engine. If you are stuck on a legacy Gulp build, it works, but modern architectures should migrate to cssnano within a PostCSS stream or use the standalone clean-css API directly.
postcss-clean:
Avoid choosing postcss-clean for new projects. This package acts as a simple bridge to run clean-css inside PostCSS but is largely redundant given cssnano's dominance and superior maintenance. It offers no significant advantage over cssnano and may introduce compatibility issues with modern PostCSS versions.
postcss-minify:
Avoid choosing postcss-minify for new projects. This package is generally considered deprecated or unmaintained compared to cssnano. It provides only basic minification capabilities without the advanced optimization presets and plugin architecture that cssnano offers, making it a risky choice for production environments.

Popular Comparisons

clean-css

cssnano

gulp-clean-css

postcss-clean

postcss-minify

README for clean-css

npm Downloads

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 10.0+ (tested on Linux, OS X, and Windows)

Install

npm install --save-dev clean-css

Use

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

new CleanCSS({
  compatibility: {
    colors: {
      hexAlpha: false, // controls 4- and 8-character hex color support
      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: true, // 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
})

Fetch option

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.

Formatting options

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`
    semicolonAfterLastProperty: false // controls removing trailing semicolons in rule; defaults to `false` - means remove
  }
})

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:

  new CleanCSS({
    format: {
      breaks: {
        afterAtRule: 2,
        afterBlockBegins: 1, // 1 is synonymous with `true`
        afterBlockEnds: 2,
        afterComment: 1,
        afterProperty: 1,
        afterRuleBegins: 1,
        afterRuleEnds: 1,
        beforeBlockEnds: 1,
        betweenSelectors: 0 // 0 is synonymous with `false`
      }
    }
  })

which will add nicer spacing between at rules and blocks.

Inlining options

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
})

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 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`,
      variableValueOptimizers: [] // controls value optimizers which are applied to variables
    }
  }
});

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
      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
    }
  }
});

Plugins

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

var myPlugin = {
  level1: {
    property: function removeRepeatedBackgroundRepeat(_rule, property, _options) {
      // So `background-repeat:no-repeat no-repeat` becomes `background-repeat:no-repeat`
      if (property.name == 'background-repeat' && property.value.length == 2 && property.value[0][1] == property.value[1][1]) {
        property.value.pop();
        property.dirty = true;
      }
    }
  }
}

new CleanCSS({plugins: [myPlugin]})

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.

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

Example: Minifying a CSS string:

const CleanCSS = require("clean-css");

const output = new CleanCSS().minify(`

  a {
    color: blue;
  }
  div {
    margin: 5px
  }

`);

console.log(output);

// Log:
{
  styles: 'a{color:#00f}div{margin:5px}',
  stats: {
    efficiency: 0.6704545454545454,
    minifiedSize: 29,
    originalSize: 88,
    timeSpent: 6
  },
  errors: [],
  inlinedStylesheets: [],
  warnings: []
}

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
});

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

var 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.

new CleanCSS({ returnPromise: true })
  .minify(source)
  .then(function (output) { console.log(output.styles); })
  .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:

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.

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.

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

How to process remote `@import`s 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.

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.

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.

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:

new CleanCSS({
  compatibility: {
    customUnits: {
      rpx: true
    }
  }
}).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.

.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}

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 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
});

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;

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:

const CleanCSS = require("clean-css");

const output = new CleanCSS().minify(`

  a {
    -notarealproperty-: 5px;
    color:
  }
  div {
    margin: 5px
  }

`);

console.log(output);

// Log:
{
  styles: 'div{margin:5px}',
  stats: {
    efficiency: 0.8695652173913043,
    minifiedSize: 15,
    originalSize: 115,
    timeSpent: 1
  },
  errors: [],
  inlinedStylesheets: [],
  warnings: [
    "Invalid property name '-notarealproperty-' at 4:8. Ignoring.",
    "Empty property 'color' at 5:8. Ignoring."
  ]
}

Example: Minify invalid CSS, resulting in one error:

const CleanCSS = require("clean-css");

const output = new CleanCSS().minify(`

  @import "idontexist.css";
  a {
    color: blue;
  }
  div {
    margin: 5px
  }

`);

console.log(output);

// Log:
{
  styles: 'a{color:#00f}div{margin:5px}',
  stats: {
    efficiency: 0.7627118644067796,
    minifiedSize: 28,
    originalSize: 118,
    timeSpent: 2
  },
  errors: [
    'Ignoring local @import of "idontexist.css" as resource is missing.'
  ],
  inlinedStylesheets: [],
  warnings: []
}

Clean-css for Gulp

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

const { src, dest, series } = require('gulp');
const CleanCSS = require('clean-css');
const concat = require('gulp-concat');

function css() {
    const options = {
        compatibility: '*', // (default) - Internet Explorer 10+ compatibility mode
        inline: ['all'], // enables all inlining, same as ['local', 'remote']
        level: 2 // Optimization levels. The level option can be either 0, 1 (default), or 2, e.g.
        // Please note that level 1 optimization options are generally safe while level 2 optimizations should be safe for most users.
    };

    return src('app/**/*.css')
        .pipe(concat('style.min.css'))
        .on('data', function(file) {
            const buferFile = new CleanCSS(options).minify(file.contents)
            return file.contents = Buffer.from(buferFile.styles)
        })
        .pipe(dest('build'))
}
exports.css = series(css)

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?

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:

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

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.