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CSSO (CSS Optimizer) is a CSS minifier. It performs three sort of transformations: cleaning (removing redundants), compression (replacement for the shorter forms) and restructuring (merge of declarations, rules and so on). As a result an output CSS becomes much smaller in size.


npm install csso


import { minify } from 'csso';
// CommonJS is also supported
// const { minify } = require('csso');

const minifiedCss = minify('.test { color: #ff0000; }').css;

// .test{color:red}

Bundles are also available for use in a browser:

  • dist/csso.js – minified IIFE with csso as global
<script src="node_modules/csso/dist/csso.js"></script>
  csso.minify('.example { color: green }');
  • dist/csso.esm.js – minified ES module
<script type="module">
  import { minify } from 'node_modules/csso/dist/csso.esm.js'

  minify('.example { color: green }');

One of CDN services like unpkg or jsDelivr can be used. By default (for short path) a ESM version is exposing. For IIFE version a full path to a bundle should be specified:

<!-- ESM -->
<script type="module">
  import * as csstree from 'https://cdn.jsdelivr.net/npm/csso';
  import * as csstree from 'https://unpkg.com/csso';

<!-- IIFE with an export to global -->
<script src="https://cdn.jsdelivr.net/npm/csso/dist/csso.js"></script>
<script src="https://unpkg.com/csso/dist/csso.js"></script>

CSSO is based on CSSTree to parse CSS into AST, AST traversal and to generate AST back to CSS. All CSSTree API is available behind syntax field extended with compress() method. You may minify CSS step by step:

import { syntax } from 'csso';

const ast = syntax.parse('.test { color: #ff0000; }');
const compressedAst = syntax.compress(ast).ast;
const minifiedCss = syntax.generate(compressedAst);

// .test{color:red}

Also syntax can be imported using csso/syntax entry point:

import { parse, compress, generate } from 'csso/syntax';

const ast = parse('.test { color: #ff0000; }');
const compressedAst = compress(ast).ast;
const minifiedCss = generate(compressedAst);

// .test{color:red}

Warning: CSSO doesn't guarantee API behind a syntax field as well as AST format. Both might be changed with changes in CSSTree. If you rely heavily on syntax API, a better option might be to use CSSTree directly.

Related projects


minify(source[, options])

Minify source CSS passed as String.

const result = csso.minify('.test { color: #ff0000; }', {
    restructure: false,   // don't change CSS structure, i.e. don't merge declarations, rulesets etc
    debug: true           // show additional debug information:
                          // true or number from 1 to 3 (greater number - more details)

// > .test{color:red}

Returns an object with properties:


  • sourceMap

    Type: Boolean
    Default: false

    Generate a source map when true.

  • filename

    Type: String
    Default: '<unknown>'

    Filename of input CSS, uses for source map generation.

  • debug

    Type: Boolean
    Default: false

    Output debug information to stderr.

  • beforeCompress

    Type: function(ast, options) or Array<function(ast, options)> or null
    Default: null

    Called right after parse is run.

  • afterCompress

    Type: function(compressResult, options) or Array<function(compressResult, options)> or null
    Default: null

    Called right after syntax.compress() is run.

  • Other options are the same as for syntax.compress() function.

minifyBlock(source[, options])

The same as minify() but for list of declarations. Usually it's a style attribute value.

const result = csso.minifyBlock('color: rgba(255, 0, 0, 1); color: #ff0000');

// > color:red

syntax.compress(ast[, options])

Does the main task – compress an AST. This is CSSO's extension in CSSTree syntax API.

NOTE: syntax.compress() performs AST compression by transforming input AST by default (since AST cloning is expensive and needed in rare cases). Use clone option with truthy value in case you want to keep input AST untouched.

Returns an object with properties:

  • ast Object – resulting AST


  • restructure

    Type: Boolean
    Default: true

    Disable or enable a structure optimisations.

  • forceMediaMerge

    Type: Boolean
    Default: false

    Enables merging of @media rules with the same media query by splitted by other rules. The optimisation is unsafe in general, but should work fine in most cases. Use it on your own risk.

  • clone

    Type: Boolean
    Default: false

    Transform a copy of input AST if true. Useful in case of AST reuse.

  • comments

    Type: String or Boolean
    Default: true

    Specify what comments to leave:

    • 'exclamation' or true – leave all exclamation comments (i.e. /*! .. */)
    • 'first-exclamation' – remove every comment except first one
    • false – remove all comments
  • usage

    Type: Object or null
    Default: null

    Usage data for advanced optimisations (see Usage data for details)

  • logger

    Type: Function or null
    Default: null

    Function to track every step of transformation.

Source maps

To get a source map set true for sourceMap option. Additianaly filename option can be passed to specify source file. When sourceMap option is true, map field of result object will contain a SourceMapGenerator instance. This object can be mixed with another source map or translated to string.

const csso = require('csso');
const css = fs.readFileSync('path/to/my.css', 'utf8');
const result = csso.minify(css, {
  filename: 'path/to/my.css', // will be added to source map as reference to source file
  sourceMap: true             // generate source map

// { css: '...minified...', map: SourceMapGenerator {} }

// '{ .. source map content .. }'

Example of generating source map with respect of source map from input CSS:

import { SourceMapConsumer } from 'source-map';
import * as csso from 'csso';

const inputFile = 'path/to/my.css';
const input = fs.readFileSync(inputFile, 'utf8');
const inputMap = input.match(/\/\*# sourceMappingURL=(\S+)\s*\*\/\s*$/);
const output = csso.minify(input, {
  filename: inputFile,
  sourceMap: true

// apply input source map to output
if (inputMap) {
    new SourceMapConsumer(inputMap[1]),

// result CSS with source map
  output.css +
  '/*# sourceMappingURL=data:application/json;base64,' +
  Buffer.from(output.map.toString()).toString('base64') +
  ' */'

Usage data

CSSO can use data about how CSS is used in a markup for better compression. File with this data (JSON) can be set using usage option. Usage data may contain following sections:

  • blacklist – a set of black lists (see Black list filtering)
  • tags – white list of tags
  • ids – white list of ids
  • classes – white list of classes
  • scopes – groups of classes which never used with classes from other groups on the same element

All sections are optional. Value of tags, ids and classes should be an array of a string, value of scopes should be an array of arrays of strings. Other values are ignoring.

White list filtering

tags, ids and classes are using on clean stage to filter selectors that contain something not in the lists. Selectors are filtering only by those kind of simple selector which white list is specified. For example, if only tags list is specified then type selectors are checking, and if all type selectors in selector present in list or selector has no any type selector it isn't filter.

ids and classes are case sensitive, tags – is not.

Input CSS:

* { color: green; }
ul, ol, li { color: blue; }
UL.foo, span.bar { color: red; }

Usage data:

    "tags": ["ul", "LI"]

Resulting CSS:


Filtering performs for nested selectors too. :not() pseudos content is ignoring since the result of matching is unpredictable. Example for the same usage data as above:

:nth-child(2n of ul, ol) { color: red }
:nth-child(3n + 1 of img) { color: yellow }
:not(div, ol, ul) { color: green }
:has(:matches(ul, ol), ul, ol) { color: blue }

Turns into:

:nth-child(2n of ul){color:red}:not(div,ol,ul){color:green}:has(:matches(ul),ul){color:blue}

Black list filtering

Black list filtering performs the same as white list filtering, but filters things that mentioned in the lists. blacklist can contain the lists tags, ids and classes.

Black list has a higher priority, so when something mentioned in the white list and in the black list then white list occurrence is ignoring. The :not() pseudos content ignoring as well.

* { color: green; }
ul, ol, li { color: blue; }
UL.foo, li.bar { color: red; }

Usage data:

    "blacklist": {
        "tags": ["ul"]
    "tags": ["ul", "LI"]

Resulting CSS:



Scopes is designed for CSS scope isolation solutions such as css-modules. Scopes are similar to namespaces and define lists of class names that exclusively used on some markup. This information allows the optimizer to move rules more agressive. Since it assumes selectors from different scopes don't match for the same element. This can improve rule merging.

Suppose we have a file:

.module1-foo { color: red; }
.module1-bar { font-size: 1.5em; background: yellow; }

.module2-baz { color: red; }
.module2-qux { font-size: 1.5em; background: yellow; width: 50px; }

It can be assumed that first two rules are never used with the second two on the same markup. But we can't say that for sure without a markup review. The optimizer doesn't know it either and will perform safe transformations only. The result will be the same as input but with no spaces and some semicolons:


With usage data CSSO can produce better output. If follow usage data is provided:

    "scopes": [
        ["module1-foo", "module1-bar"],
        ["module2-baz", "module2-qux"]

The result will be (29 bytes extra saving):


If class name isn't mentioned in the scopes it belongs to default scope. scopes data doesn't affect classes whitelist. If class name mentioned in scopes but missed in classes (both sections are specified) it will be filtered.

Note that class name can't be set for several scopes. Also a selector can't have class names from different scopes. In both cases an exception will thrown.

Currently the optimizer doesn't care about changing order safety for out-of-bounds selectors (i.e. selectors that match to elements without class name, e.g. .scope div or .scope ~ :last-child). It assumes that scoped CSS modules doesn't relay on it's order. It may be fix in future if to be an issue.

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