<h1 align="center">
<br/>
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</h1>
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clean-css is a fast and efficient CSS optimizer for [Node.js](http://nodejs.org/) platform and [any modern browser](https://jakubpawlowicz.github.io/clean-css).
According to [tests](http://goalsmashers.github.io/css-minification-benchmark/) it is one of the best available.
**Table of Contents**
- [Node.js version support](#nodejs-version-support)
- [Install](#install)
- [Use](#use)
* [Important: 4.0 breaking changes](#important-40-breaking-changes)
* [What's new in version 4.1](#whats-new-in-version-41)
* [What's new in version 4.2](#whats-new-in-version-42)
* [Constructor options](#constructor-options)
* [Compatibility modes](#compatibility-modes)
* [Fetch option](#fetch-option)
* [Formatting options](#formatting-options)
* [Inlining options](#inlining-options)
* [Optimization levels](#optimization-levels)
+ [Level 0 optimizations](#level-0-optimizations)
+ [Level 1 optimizations](#level-1-optimizations)
+ [Level 2 optimizations](#level-2-optimizations)
* [Minify method](#minify-method)
* [Promise interface](#promise-interface)
* [CLI utility](#cli-utility)
- [FAQ](#faq)
* [How to optimize multiple files?](#how-to-optimize-multiple-files)
* [How to process remote `@import`s correctly?](#how-to-process-remote-imports-correctly)
* [How to apply arbitrary transformations to CSS properties?](#how-to-apply-arbitrary-transformations-to-css-properties)
* [How to specify a custom rounding precision?](#how-to-specify-a-custom-rounding-precision)
* [How to keep a CSS fragment intact?](#how-to-keep-a-css-fragment-intact)
* [How to preserve a comment block?](#how-to-preserve-a-comment-block)
* [How to rebase relative image URLs?](#how-to-rebase-relative-image-urls)
* [How to work with source maps?](#how-to-work-with-source-maps)
* [How to apply level 1 & 2 optimizations at the same time?](#how-to-apply-level-1--2-optimizations-at-the-same-time)
* [What level 2 optimizations do?](#what-level-2-optimizations-do)
* [How to use clean-css with build tools?](#how-to-use-clean-css-with-build-tools)
* [How to use clean-css from web browser?](#how-to-use-clean-css-from-web-browser)
- [Contributing](#contributing)
* [How to get started?](#how-to-get-started)
- [Acknowledgments](#acknowledgments)
- [License](#license)
# Node.js version support
clean-css requires Node.js 4.0+ (tested on Linux, OS X, and Windows)
# Install
```
npm install --save-dev clean-css
```
# Use
```js
var CleanCSS = require('clean-css');
var input = 'a{font-weight:bold;}';
var options = { /* options */ };
var output = new CleanCSS(options).minify(input);
```
## Important: 4.0 breaking changes
clean-css 4.0 introduces some breaking changes:
* API and CLI interfaces are split, so API stays in this repository while CLI moves to [clean-css-cli](https://github.com/jakubpawlowicz/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.
## 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](https://jakubpawlowicz.github.io/clean-css) gets an improved settings panel with "reset to defaults", instant option changes, and settings being persisted across sessions.
## 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](#how-to-apply-arbitrary-transformations-to-css-properties);
* adds configurable line breaks via `format: { breakWith: 'lf' }` option.
## 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](#compatibility-modes) for examples;
* `fetch` - controls a function for handling remote requests; see [fetch option](#fetch-option) for examples (since 4.1.0);
* `format` - controls output CSS formatting; defaults to `false`; see [formatting options](#formatting-options) for examples;
* `inline` - controls `@import` inlining rules; defaults to `'local'`; see [inlining options](#inlining-options) for examples;
* `inlineRequest` - controls extra options for inlining remote `@import` rules, can be any of [HTTP(S) request options](https://nodejs.org/api/http.html#http_http_request_options_callback);
* `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](#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](#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](https://github.com/jakubpawlowicz/clean-css/blob/master/lib/options/compatibility.js), with the following options available:
```js
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.
```js
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.
```js
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](https://github.com/jakubpawlowicz/clean-css/blob/master/lib/reader/load-remote-resource.js) 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:
```js
new CleanCSS({
format: 'beautify' // formats output in a really nice way
})
```
and
```js
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:
```js
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`
}
})
```
## Inlining options
`inline` option whitelists which `@import` rules will be processed, e.g.
```js
new CleanCSS({
inline: ['local'] // default; enables local inlining only
})
```
```js
new CleanCSS({
inline: ['none'] // disables all inlining
})
```
```js
// introduced in clean-css 4.1.0
new CleanCSS({
inline: false // disables all inlining (alias to `['none']`)
})
```
```js
new CleanCSS({
inline: ['all'] // enables all inlining, same as ['local', 'remote']
})
```
```js
new CleanCSS({
inline: ['local', 'mydomain.example.com'] // enables local inlining plus given remote source
})
```
```js
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.
```js
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:
```js
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.
```js
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:
```js
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.
```js
new CleanCSS({
level: {
2: {
all: false, // sets all values to `false`
removeDuplicateRules: true // turns on removing duplicate rules
}
}
});
```
## Minify method
Once configured clean-css provides a `minify` method to optimize a given CSS, e.g.
```js
var output = new CleanCSS(options).minify(source);
```
The output of the `minify` method is a hash with following fields:
```js
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.
```js
var output = new CleanCSS(options).minify(source, inputSourceMap);
```
or a callback invoked when optimizations are finished, e.g.
```js
new CleanCSS(options).minify(source, function (error, output) {
// `output` is the same as in the synchronous call above
});
```
## Promise interface
If you prefer clean-css to return a Promise object then you need to explicitely ask for it, e.g.
```js
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/jakubpawlowicz/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:
```js
new CleanCSS().minify(['path/to/file/one', 'path/to/file/two']);
```
```js
new CleanCSS().minify({
'path/to/file/one': {
styles: 'contents of file one'
},
'path/to/file/two': {
styles: 'contents of file two'
}
});
```
```js
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](http://2ality.com/2015/10/property-traversal-order-es6.html) - available since 4.1.0.
Important note - any `@import` rules already present in the hash will be resolved in memory.
## 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.:
```js
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 `@import`s will be left as is.
## How to apply arbitrary transformations to CSS properties?
If clean-css doesn't perform a particular property optimization, you can use `transform` callback to apply it:
```js
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.
## How to specify a custom rounding precision?
The level 1 `roundingPrecision` optimization option accept a string with per-unit rounding precision settings, e.g.
```js
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 keep a CSS fragment intact?
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.
```css
.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:
```css
.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 `/*`:
```css
/*!
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.:
```js
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:
```js
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:
```js
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.
```js
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](https://github.com/jakubpawlowicz/clean-css/blob/master/lib/optimizer/level-2/optimize.js#L67), and this is what they do:
* `recursivelyOptimizeBlocks` - does all the following operations on a nested block, like `@media` or `@keyframe`;
* `recursivelyOptimizeProperties` - optimizes properties in rulesets and flat at-rules, like @font-face, by splitting them into components (e.g. `margin` into `margin-(bottom|left|right|top)`), optimizing, and restoring them back. You may want to use `mergeIntoShorthands` option to control whether you want to turn multiple components into shorthands;
* `removeDuplicates` - gets rid of duplicate rulesets with exactly the same set of properties, e.g. when including a Sass / Less partial twice for no good reason;
* `mergeAdjacent` - merges adjacent rulesets with the same selector or rules;
* `reduceNonAdjacent` - identifies which properties are overridden in same-selector non-adjacent rulesets, and removes them;
* `mergeNonAdjacentBySelector` - identifies same-selector non-adjacent rulesets which can be moved (!) to be merged, requires all intermediate rulesets to not redefine the moved properties, or if redefined to have the same value;
* `mergeNonAdjacentByBody` - same as the one above but for same-selector non-adjacent rulesets;
* `restructure` - tries to reorganize different-selector different-rules rulesets so they take less space, e.g. `.one{padding:0}.two{margin:0}.one{margin-bottom:3px}` into `.two{margin:0}.one{padding:0;margin-bottom:3px}`;
* `removeDuplicateFontAtRules` - removes duplicated `@font-face` rules;
* `removeDuplicateMediaQueries` - removes duplicated `@media` nested blocks;
* `mergeMediaQueries` - merges non-adjacent `@media` at-rules by the same rules as `mergeNonAdjacentBy*` above;
## How to use clean-css with build tools?
There is a number of 3rd party plugins to popular build tools:
* [Broccoli](https://github.com/broccolijs/broccoli#broccoli): [broccoli-clean-css](https://github.com/shinnn/broccoli-clean-css)
* [Brunch](http://brunch.io/): [clean-css-brunch](https://github.com/brunch/clean-css-brunch)
* [Grunt](http://gruntjs.com): [grunt-contrib-cssmin](https://github.com/gruntjs/grunt-contrib-cssmin)
* [Gulp](http://gulpjs.com/): [gulp-clean-css](https://github.com/scniro/gulp-clean-css)
* [Gulp](http://gulpjs.com/): [using vinyl-map as a wrapper - courtesy of @sogko](https://github.com/jakubpawlowicz/clean-css/issues/342)
* [component-builder2](https://github.com/component/builder2.js): [builder-clean-css](https://github.com/poying/builder-clean-css)
* [Metalsmith](http://metalsmith.io): [metalsmith-clean-css](https://github.com/aymericbeaumet/metalsmith-clean-css)
* [Lasso](https://github.com/lasso-js/lasso): [lasso-clean-css](https://github.com/yomed/lasso-clean-css)
* [Start](https://github.com/start-runner/start): [start-clean-css](https://github.com/start-runner/clean-css)
## How to use clean-css from web browser?
* https://jakubpawlowicz.github.io/clean-css/ (official web interface)
* http://refresh-sf.com/
* http://adamburgess.github.io/clean-css-online/
# Contributing
See [CONTRIBUTING.md](https://github.com/jakubpawlowicz/clean-css/blob/master/CONTRIBUTING.md).
## How to get started?
First clone the sources:
```bash
git clone git@github.com:jakubpawlowicz/clean-css.git
```
then install dependencies:
```bash
cd clean-css
npm install
```
then use any of the following commands to verify your copy:
```bash
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
```
# Acknowledgments
Sorted alphabetically by GitHub handle:
* [@abarre](https://github.com/abarre) (Anthony Barre) for improvements to `@import` processing;
* [@alexlamsl](https://github.com/alexlamsl) (Alex Lam S.L.) for testing early clean-css 4 versions, reporting bugs, and suggesting numerous improvements.
* [@altschuler](https://github.com/altschuler) (Simon Altschuler) for fixing `@import` processing inside comments;
* [@ben-eb](https://github.com/ben-eb) (Ben Briggs) for sharing ideas about CSS optimizations;
* [@davisjam](https://github.com/davisjam) (Jamie Davis) for disclosing ReDOS vulnerabilities;
* [@facelessuser](https://github.com/facelessuser) (Isaac) for pointing out a flaw in clean-css' stateless mode;
* [@grandrath](https://github.com/grandrath) (Martin Grandrath) for improving `minify` method source traversal in ES6;
* [@jmalonzo](https://github.com/jmalonzo) (Jan Michael Alonzo) for a patch removing node.js' old `sys` package;
* [@lukeapage](https://github.com/lukeapage) (Luke Page) for suggestions and testing the source maps feature;
Plus everyone else involved in [#125](https://github.com/jakubpawlowicz/clean-css/issues/125) for pushing it forward;
* [@madwizard-thomas](https://github.com/madwizard-thomas) for sharing ideas about `@import` inlining and URL rebasing.
* [@ngyikp](https://github.com/ngyikp) (Ng Yik Phang) for testing early clean-css 4 versions, reporting bugs, and suggesting numerous improvements.
* [@wagenet](https://github.com/wagenet) (Peter Wagenet) for suggesting improvements to `@import` inlining behavior;
* [@venemo](https://github.com/venemo) (Timur Kristóf) for an outstanding contribution of advanced property optimizer for 2.2 release;
* [@vvo](https://github.com/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](https://github.com/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](https://github.com/jakubpawlowicz/clean-css/blob/master/LICENSE).
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