Using Less

Using less in node, browser and third party. For general installation instructions and an overview, please read the Using Less section on our front page.

Edit the markdown source for "v2-upgrade"

Language Changes

Colours now output as they are written, so purple stays as purple and is not converted to its hex representation.

Command Line Usage

Clean CSS

We have removed the dependency on clean css and moved it to a plugin. This allows us to:

  1. update the dependency and integration without a less release
  2. not tie people who do not use clean css into having it downloaded by npm.

The usage is similar, just install the plugin (npm install -g less-plugin-clean-css) then tell less to use it by using the --clean-css argument.

# old
lessc --clean-css --clean-option=--compatibility:ie8 --clean-option=--advanced
# new
lessc --clean-css="--compatibility=ie8 --advanced"


We have improved the source map options and path generation so the sourcemap should be generated at the correct path without specifying any options.

Programmatic Usage

We have deprecated the use of less.Parser and toCss to generate the css. Instead we require you to use the less.render shorthand. See Programmatic Usage for more information.

Further, instead of returning a string which is the css, we return an object with a css field set to the string and a map field set to the sourcemap (if applicable).

The sourcemap options are now to be set on sourceMap instead of directly on options. So instead of options.sourceMapFullFilename = you would set options.sourceMap = { sourceMapFullFilename:.

Browser Usage

The browser usage has not changed significantly. Options set on the less object are exposed as less.options after the less script has run, rather than polluting less.

It is now possible to specify options on the script and less tags, which should simplify option setting in the browser. See the browser usage section for more information.

Edit the markdown source for "command-line-usage"

Compile .less files to .css using the command line

Heads up! If the command line isn't your thing, learn more about GUIs for Less.

Installing lessc for Use Globally

Install with npm

npm install less -g

and then you will have the lessc command available globally. For a specific version (or tag) you can add @VERSION after our package name, e.g. npm install less@1.6.2 -g.

Installing for Node Development

Alternatively if you don't use the compiler globally, you may be after

npm i less --save-dev

This will install the latest official version of lessc in your project folder, also adding it to the devDependencies in your project's package.json.

Note that a caret version range will be automatically specified in package.json. This is good, as new minor releases of the latest version will be installable by npm.

Beta releases of lessc

Periodically, as new functionality is being developed, lessc builds will be published to npm, tagged as beta. These builds will not be published as a @latest official release, and will typically have beta in the version (use lessc -v to get current version).

Since patch releases are non-breaking we will publish patch releases immediately and alpha/beta/candidate versions will be published as minor or major version upgrades (we endeavour since 1.4.0 to follow semantic versioning).

Installing an unpublished development version of lessc

If you want to install a bleeding-edge, unpublished version of lessc, follow the instructions for specifying a git URL as a dependency and be sure to specify an actual commit SHA (not a branch name) as the commit-ish. This will guarantee that your project always uses that exact version of lessc.

The specified git URL may be that of the official lessc repo or a fork.

Server-Side and Command Line Usage

The binary included in this repository, bin/lessc works with Node.js on *nix, OS X and Windows.

Usage: lessc [option option=parameter ...] <source> [destination]

Command Line Usage

lessc [option option=parameter ...] <source> [destination]

If source is set to `-' (dash or hyphen-minus), input is read from stdin.


# compile bootstrap.less to bootstrap.css
$ lessc bootstrap.less bootstrap.css

# compile bootstrap.less to bootstrap.css and minify (compress) the result
$ lessc -x bootstrap.less bootstrap.css



lessc --help
lessc -h

Prints a help message with available options and exits.

Include Paths

lessc --include-path=PATH1;PATH2

Sets available include paths. Separated by ':' or ';' on Windows.

Use this to configure a list of paths which less will use to find imports in. You might use this for instance to specify a path to a library which you want to be referenced simply and relatively in the less files.

In node, set a paths option

{ paths: ['PATH1', 'PATH2']  }


lessc -M
lessc --depends

No Color

lessc --no-color

No IE Compatibility

lessc --no-ie-compat

Currently only used for the data-uri function to ensure that images aren't created that are too large for the browser to handle.

Disable JavaScript

lessc --no-js


lessc --lint
lessc -l

Runs the less parser and just reports errors without any output.


lessc -s
lessc --silent

Stops any warnings from being shown.

Strict Imports

lessc --strict-imports

Allow Imports from Insecure HTTPS Hosts

lessc --insecure


lessc -v
lessc --version


lessc -x
lessc --compress

Compress using less built-in compression. This does an okay job but does not utilise all the tricks of dedicated css compression. Please feel free to improve our compressed output with a pull request.

Clean CSS

In v2 of less, Clean CSS is no longer included as a direct dependency. To use clean css with lessc, use the clean css plugin.

Source Map Output Filename

lessc --source-map

Tells less to generate a sourcemap. If you have the sourcemap option without a filename it will use the source less file name but with the extension map.

Source Map Rootpath

lessc --source-map-rootpath=dev-files/

Specifies a rootpath that should be prepended to each of the less file paths inside the sourcemap and also to the path to the map file specified in your output css.

Because the basepath defaults to the directory of the input less file, the rootpath defaults to the path from the sourcemap output file to the base directory of the input less file.

Use this option if for instance you have a css file generated in the root on your web server but have your source less/css/map files in a different folder. So for the option above you might have


Source Map Basepath

lessc --source-map-basepath=less-files/

This is the opposite of the rootpath option, it specifies a path which should be removed from the output paths. For instance if you are compiling a file in the less-files directory but the source files will be available on your web server in the root or current directory, you can specify this to remove the additional less-files part of the path.

It defaults to the path to the input less file.

Source Map Less Inline

lessc --source-map-less-inline

This option specifies that we should include all of the Less files in to the sourcemap. This means that you only need your map file to get to your original source.

This can be used in conjunction with the map inline option so that you do not need to have any additional external files at all.

Source Map Map Inline

lessc --source-map-map-inline

This option specifies that the map file should be inline in the output CSS. This is not recommended for production, but for development it allows the compiler to produce a single output file which in browsers that support it, use the compiled css but show you the non-compiled less source.

Source Map URL

lessc --source-map-url=../my-map.json

Allows you to override the URL in the css that points at the map file. This is for cases when the rootpath and basepath options are not producing exactly what you need.


lessc -rp=resources/
lessc --rootpath=resources/

Allows you to add a path to every generated import and url in your css. This does not affect less import statements that are processed, just ones that are left in the output css.

For instance, if all the images the css use are in a folder called resources, you can use this option to add this on to the URL's and then have the name of that folder configurable.

Relative URLs

lessc -ru
lessc --relative-urls

By default URLs are kept as-is, so if you import a file in a sub-directory that references an image, exactly the same URL will be output in the css. This option allows you to re-write URL's in imported files so that the URL is always relative to the base imported file. E.g.

# main.less
@import "files/backgrounds.less";
# files/backgrounds.less
.icon-1 {
  background-image: url('images/lamp-post.png');

this will output the following normally

.icon-1 {
  background-image: url('images/lamp-post.png');

but with this option on it will instead output

.icon-1 {
  background-image: url('files/images/lamp-post.png');

You may also want to consider using the data-uri function instead of this option, which will embed images into the css.

Strict Math

lessc -sm=on
lessc --strict-math=on

Defaults to Off.

Without this option on Less will try and process all maths in your css e.g.

.class {
  height: calc(100% - 10px);

will be processed currently.

With strict math on, only maths that is inside un-necessary parenthesis will be processed. For instance.

.class {
  width: calc(100% - (10px  - 5px));
  height: (100px / 4px);
  font-size: 1 / 4;
.class {
  width: calc(100% - 5px);
  height: 25px;
  font-size: 1 / 4;

We originally planned to default this to true in the future, but it has been a controversial option and we are considering whether we have solved the problem in the right way, or whether less should just have exceptions for instances where / is valid or calc is used.

Strict Units

lessc -su=on
lessc --strict-units=on

Defaults to off.

Without this option, less attempts to guess at the output unit when it does maths. For instance

.class {
  property: 1px * 2px;

In this case, things are clearly not right - a length multiplied by a length gives an area, but css does not support specifying areas. So we assume that the user meant for one of the values to be a value, not a unit of length and we output 2px.

With strict units on, we assume this is a bug in the calculation and throw an error.

Global Variable

lessc --global-var="my-background=red"

This option defines a variable that can be referenced by the file. Effectively the declaration is put at the top of your base Less file, meaning it can be used but it also can be overridden if this variable is defined in the file.

Modify Variable

lessc --modify-var="my-background=red"

As opposed to the global variable option, this puts the declaration at the end of your base file, meaning it will override anything defined in your Less file.

URL Arguments

lessc --url-args="cache726357"

This option allows you to specify a argument to go on to every URL. This may be used for cache-busting for instance.

Line Numbers

lessc --line-numbers=comments
lessc --line-numbers=mediaquery
lessc --line-numbers=all

Generates inline source-mapping. This was the only option before browsers started supporting sourcemaps. We are consider deprecating, so please get in touch if you want this option to stick around.


lessc --clean-css
lessc --plugin=clean-css="advanced"

--plugin Loads a plugin. You can also omit the --plugin= if the plugin begins less-plugin. E.g. the clean css plugin is called less-plugin-clean-css once installed (npm install less-plugin-clean-css), use either with --plugin=less-plugin-clean-css or just --clean-css specify options afterwards e.g. --plugin=less-plugin-clean-css="advanced" or --clean-css="advanced"

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We recommend using less in the browser only for development or when you need to dynamically compile less and cannot do it serverside. This is because less is a large javascript file and compiling less before the user can see the page means a delay for the user. In addition, consider that mobile devices will compile slower. For development consider if using a watcher and live reload (e.g. with grunt or gulp) would be better suited.

To use less in the browser, you firstly need to include the less script.

<!-- Here: include any less plugin scripts, any required browser shims and optionally set less = any options  -->
<script src="less.js"></script>

This will find any less style tags on the page

<link rel="stylesheet/less" type="text/css" href="styles.less" />

and create style tags with the compiled css synchronously.

Setting Options

You can set options either programmatically, by setting them on a less object before the script tag - this then effects all initial link tags and programmatic usage of less.

  less = {
    env: "development"
<script src="less.js"></script>

The other way is to specify the options on the script tag, e.g.

  less = {
    env: "development"
<script src="less.js" data-env="development"></script>

And you can also do this on link tags to override certain settings (some less settings like verbose are global and can not be overridden).

<link data-dump-line-numbers="all" data-global-vars='{ "myvar": "#ddffee", "mystr": "\"quoted\"" }' rel="stylesheet/less" type="text/css" href="less/styles.less">

The important points for attribute options are..

  • importance level: window.less < script tag < link tag
  • data attributes names are not camelCase (e.g logLevel -> data-log-level)
  • link tag options are just render time options (e.g verbose, logLevel ... are not supported)
  • non-string data attributes values should be JSON valid (e.g use double quotes instead of single quotes like in data-global-vars='{ "myvar": "#ddffee", "mystr": "\"quoted\"" }')

Watch Mode

To enable Watch mode, option env must be set to development. Then AFTER the less.js file is included, call, like this:

<script>less = { env: 'development'};</script>
<script src="less.js"></script>

Alternatively, you can enable Watch mode temporarily by appending #!watch to the URL.

Modify Variables

Enables run-time modification of Less variables. When called with new values, the Less file is recompiled without reloading. Simple basic usage:

  '@buttonFace': '#5B83AD',
  '@buttonText': '#D9EEF2'


It is possible to output rules in your CSS which allow tools to locate the source of the rule.

Either specify the option dumpLineNumbers as above or add !dumpLineNumbers:mediaquery to the url.

You can use the mediaquery option with FireLESS (it is identical to the SCSS media query debugging format). Also see FireLess and Less v2. The comment option can be used to display file information and line numbers in the inline compiled CSS code.


Set options in a global less object before loading the less.js script:

<!-- set options before less.js script -->
  less = {
    env: "development",
    logLevel: 2,
    async: false,
    fileAsync: false,
    poll: 1000,
    functions: {},
    dumpLineNumbers: "comments",
    relativeUrls: false,
    globalVars: {
      var1: '"string value"',
      var2: 'regular value'
    rootpath: ":/"
<script src="less.js"></script>


Type: Boolean

Default: false

Whether to request the import files with the async option or not. See fileAsync.


Type: String Options: ''| 'comments'|'mediaquery'|'all' Default: ''

When set, this adds source line information to the output css file. This helps you debug where a particular rule came from.

The comments option is used for outputting user-understandable content, whilst mediaquery is for use with a firefox extension which parses the css and extracts the information.

In the future we hope this option to be superseded by sourcemaps.


Type: String Default: depends on page URL

Environment to run may be either development or production.

In production, your css is cached in local storage and information messages are not output to the console.

If the document's URL starts with file:// or is on your local machine or has a non standard port, it will automatically be set to development.


less = { env: 'production' };


Type: String

Options: html|console|function

Default: html

Set the method of error reporting when compilation fails.


Type: Boolean

Default: false

Whether to request the import asynchronously when in a page with a file protocol.


Type: object

User functions, keyed by name.


less = {
    functions: {
        myfunc: function() {
            return new(less.tree.Dimension)(1);

and it can be used like a native Less function e.g.

.my-class {
  border-width: unit(myfunc(), px);


Type: Number

Default: 2

The amount of logging in the javascript console. Note: If you are in the production environment you will not get any logging.

2 - Information and errors
1 - Errors
0 - Nothing


Type: Integer

Default: 1000

The amount of time (in milliseconds) between polls while in watch mode.


Type: Boolean

Default: false

Optionally adjust URLs to be relative. When false, URLs are already relative to the entry less file.


Type: Object

Default: undefined

List of global variables to be injected into the code. Keys of the object are variables names, values are variables values. Variables of "string" type must explicitly include quotes if needed.


less.globalVars = { myvar: "#ddffee", mystr: "\"quoted\"" };

This option defines a variable that can be referenced by the file. Effectively the declaration is put at the top of your base Less file, meaning it can be used but it also can be overridden if this variable is defined in the file.


Type: Object

Default: undefined

Same format as globalVars.

As opposed to the globalVars option, this puts the declaration at the end of your base file, meaning it will override anything defined in your Less file.


Type: String

Default: false

A path to add on to the start of every URL resource.


Type: Boolean

Default: true (previously false in before v2)

Whether to use the per session file cache. This caches less files so that you can call modifyVars and it will not retrieve all the less files again. If you use the watcher or call refresh with reload set to true, then the cache will be cleared before running.

Edit the markdown source for "browser-support"

Less only supports running on modern browsers (recent versions of Chrome, Firefox, Safari and IE). While it is possible to use LESS on the client side in production, please be aware that there are performance implications for doing so (although the latest releases of LESS are quite a bit faster). Also, sometimes cosmetic issues can occur if a JavaScript error occurs. This is a trade off of flexibility vs. speed. For the fastest performance possible for a static web site, we recommend compiling LESS on the server side.

Note that PhantomJS does not currently implement Function.prototype.bind so you will require a es-5 shim for this function to run under PhantomJS (We use PhantomJS for tests and we append an es5-shim to make it work).

There are reasons to use client-side less in production, such as if you want to allow users to tweak variables which will affect the theme and you want to show it to them in real-time - in this instance a user is not worried about waiting for a style to update before seeing it.

If you need to run less in an older browser, please use an es-5 shim which will add the javascript features that less requires.

In addition, if you use options as attributes on the script or link tags, you will require browser support for JSON.parse or an appropriate polyfill.

Edit the markdown source for "plugins"

How do I use a plugin?

Command Line

If you are using lessc, the first thing you need to do is install that plugin. We recommend the plugin starts with "less-plugin" though that isn't required. For the clean-css plugin you would install with

npm install less-plugin-clean-css

To use the plugin, if you specify a unrecognised option, we attempt to load that, for example

lessc --clean-css="advanced"

Will use the plugin you just installed. You can also be more direct, for example

lessc --plugin=path_to_plugin=options

Using a Plugin in Code

In Node, require the plugin and pass it to less in an array as an option plugins. E.g.

var myPlugin = require("my-plugin");
less.render(myCSS, { plugins: [myPlugin] })
   .then(function(output) {
    function(error) {

In the Browser

Plugin authors should provide a javascript file, just include that in the page before the less.js script.

<script src="plugin.js"></script>
less = { 
    plugins: [plugin]
<script src="less.min.js"></script>

List of Less Plugins

Available Less plugins. Find more at the NPM Registry

Postprocessor/Feature Plugins

Autoprefixer Add vendor prefixes
CSScomb Beautify/format
clean-css Compress/minify
CSSWring Compress/minify
css-flip Generate left-to-right (LTR) or right-to-left (RTL) CSS
functions Write custom Less functions in Less itself
glob Globbing support in Less imports
group-css-media-queries Group CSS media queries
inline-urls Convert url() to a call to data-uri()
npm-import Import from npm packages
pleeease Postprocess using pleeease
rtl Reverse from ltr to rtl

Framework/Library Importers

Bootstrap import Bootstrap
Bower Resolve import from a Bower package
Cardinal CSS for less.js import Cardinal
Flexbox Grid import Flexbox Grid
Flexible Grid System import Flexible Grid System
Ionic import Ionic
Lesshat import Lesshat
Skeleton import Skeleton

Function Libraries

advanced-color-functions Some advanced colour functions that helps in finding more contrasting
cubehelix cubehelix(y,a,b,t) function returns a color between the two colors a and b, using a gamma correction value of 1
lists Lists/arrays manipulation functions library

For Plugin Authors

Less supports some entry points that allow an author to integrate with less. We may add some more in the future.

The plugin itself has a very simple signature, like this

    install: function(less, pluginManager) {
    minVersion: [2, 0, 0] /* optional */

So, the plugin gets the less object, which in v2 has more classes on it (making it easy to extend), a plugin manager which provides some hooks to add visitors, file managers and post processors.

If your plugin supports lessc, there are a few more details and the signature looks like this

    install: function(less, pluginManager) {
    setOptions: function(argumentString) { /* optional */
    printUsage: function() { /* optional */
    minVersion: [2, 0, 0] /* optional */

The additions are the setOptions function which passes the string the user enters when specifying your plugin and also the printUsage function which you should use to explain your options and how the plugin works.

Here are some example repos showing the different plugin types:

Note: Plugins are different from creating a version of less for a different environment but they do have similarities, for example node provides 2 file managers by default and browser provides one and that is the main step in getting less to run within a specific environment. The plugin allows you to add file managers.

Edit the markdown source for "programmatic-usage"

The main entry point into less is the less.render function. This takes the following format

less.render(lessInput, options)
    .then(function(output) {
        // output.css = string of css
        // = string of sourcemap
        // output.imports = array of string filenames of the imports referenced
    function(error) {

// or...

less.render(css, options, function(error, output) {})

The options argument is optional. If you specify a callback then a promise will not be returned, where as if you do not specify a callback a promise will be given. Under the hood, the callback version is used so that less can be used synchronously.

If you wanted to render a file, you would first read it into a string (to pass to less.render) and then set the filename field on options to be the filename of the main file. less will handle all the processing of the imports.

The sourceMap option is an object which enables you to set sub sourcemap options. Available sub options are: sourceMapURL,sourceMapBasepath,sourceMapRootpath,outputSourceFiles and sourceMapFileInline. Notice that the sourceMap option is not available for the less.js in browser compiler now.

    .then(function(output) {
        // output.css = string of css
        // = undefined
less.render(lessInput, {sourceMap: {}})
    .then(function(output) {
        // output.css = string of css
        // = string of sourcemap
less.render(lessInput, {sourceMap: {sourceMapFileInline: true}})
    .then(function(output) {
        // output.css = string of css \n /*# sourceMappingURL=data:application/json;base64,eyJ2ZXJ..= */
        // = undefined

Previously we also recommended creating a less.Parser and then calling toCSS on the result. However this had 2 serious drawbacks - it meant that our parser was in fact tied to all of less and 2nd it meant that the toCSS call had to be synchronous.

You can still get the less parse tree, but it requires more steps. You can see how this is done in the render function but we do not support using less in this way and may change this function in a minor release version bump (we will not break it in a patch release).

Getting Access to the Log

You can add a log listener with the following code

    debug: function(msg) {
    info: function(msg) {
    warn: function(msg) {
    error: function(msg) {

Note: all functions are optional. An error will not be logged, but instead is passed back to the callback or promise in less.render

Edit the markdown source for "online-less-compilers"

Online Integrated Development Environment (IDE) that is hosted in a browser allowing users to edit and compile Less to CSS in real-time.

This Online Less Compiler can help you to learn Less. You can go through the examples below or try your own Less code.

Online compiler for Less CSS.

A simple Less CSS file converter using the Less JS project.

less.php live demo.

lessphp live demo.


A real time preprocesser compiler.


Online Less compiler providing live demo for Less and est. Allowing users to switch among all versions of Less after 1.4.0 with optional est/Autoprefixer functionalities.


Live demo of ILess PHP compiler

Online Web IDEs/Playgrounds with Less support

CSSDeck Labs

CSSDeck Labs is a place where you can quickly create some experiments (or testcases) that involves HTML, CSS, JS code.


CodePen is a playground for the front end side of the web.

Fiddle Salad

An online playground with an instantly ready coding environment.

JS Bin

JS Bin is a webapp specifically designed to help JavaScript and CSS folk test snippets of code.


Online Web Editor

Edit the markdown source for "guis-for-less"

Tip: try out the different Less tools available for your platform to see which one meets your needs.

This page focuses on GUI compilers. For command line usage and tools see Command Line Usage.

Cross platform


Crunch is not just a Less compiler, it is also a Less editor for Windows and Mac.

If you work a lot with Less files, you should definitely try it out. Crunch is built on the Adobe AIR platform. Get more info:

Crunch screenshot


A rapid prototyping and static site generation tool for designers and developers

Mixture brings together a collection of awesome tools and best practices. It's quick, no-fuss, super-powerful and works with your favourite editor.

Get more info:

mixture screenshot


SimpLESS is a minimalistic Less compiler. Just drag, drop and compile.

One of the unique features of SimpLESS is that it supports 'prefixing' your CSS by using SimpLESS is built on the Titanium platform. Get more info:

SimpLESS screenshot


Koala is a cross-platform GUI application for compiling less, sass and coffeescript.

Features: cross platform, compile error notification supports and compile options supports.

Get more info:

koala screenshot

Specific platforms



Prepros is a tool to compile LESS, Sass, Compass, Stylus, Jade and much more.

Get more info at

Prepros screenshot


WinLess started out as a clone of, it takes a more feature-complete approach and has several settings. It also supports starting with command line arguments.

Get more info:

WinLess screenshot



CodeKit is the successor to, and supports Less among many other preprocessing languages, such as SASS, Jade, Markdown, and many more.

Get more info:

CodeKit screenshot


CSS edits and image changes apply live. CoffeeScript, SASS, Less and others just work.

Get more info:

LiveReload screenshot



Plessc is a gui fronted made with PyQT.

Auto compile, log viewer, open the less file with the editor chosen, settings for compile the file. Get more info:

Plessc screenshot

Edit the markdown source for "editors-and-plugins"

Also see: GUIs for Less.js

Editors and IDEs

Sublime Text 2 & 3


Visual Studio


Notepad++ 6.x

Edit the markdown source for "third-party-compilers"

Node.js Compilers

  • grunt-contrib-less
  • assemble-less: Full-featured Grunt.js plugin for compiling Less files to CSS, with additional options for maintaining libraries of Less components and themes. For advanced users, this plugin allows you to define and manage Less "packages" or "bundles" using JSON, Lo-dash(underscore) templates (e.g. <%= bootstrap.less %>), and node-glob / minimatch (e.g. '../src/**/*.less"). assemble-less also has a number of options including minifying CSS
  • gulp-less: Please note that this plugin discards source-map options, opting to instead using the gulp-sourcemaps library.
  • RECESS: Twitter's code quality tool for CSS built on top of Less. RECESS has options for compiling Less to CSS, as well as linting, formatting and minifying CSS.
  • autoless: A Less files watcher, with dependency tracking (changes to imported files cause other files to be updated too) and growl notifications.
  • Connect Middleware for Less.js: Connect Middleware for Less.js compiling

Other Technologies

Wro4j Runner CLI Download the wro4j-runner.jar file and run the following command:

java -jar wro4j-runner-1.5.0-jar-with-dependencies.jar --preProcessors lessCss

More details can be found here: Wro4j Runner CLI

CSS::LESSp styles.less > styles.css

Windows Script Host

Note - the official Less node runs on windows, so we are not sure why you would use this.

Less.js for Windows with this usage:

cscript //nologo lessc.wsf input.less [output.css] [-compress]


lessc input.less [output.css] [-compress]


dotless for Windows can be run like this:

dotless.Compiler.exe [-switches] <inputfile> [outputfile]

Also see:

Edit the markdown source for "frameworks-using-less"

UI/Theme Frameworks and Components

1pxdeep Brings designing by relative visual weight, or designing with color schemes, to Bootstrap
Bootflat Open source Flat UI KIT based on Bootstrap
BootPress A PHP Framework with a built in Blog and Flat File CMS
Bootstrap Front-end framework for developing responsive, mobile first projects on the web
Bootstrap a11y theme Makes web accessibility easier for Bootstrap developers
Bootswatch Collection of free themes for Bootstrap
Cardinal Small "mobile first" CSS framework for front-end developers who build responsive web applications
CSSHórus Library for development of responsive and mobile websites
Flat UI Free Theme and framework for Bootstrap
frontsize CSS front end framework
InContent Image content with description created with CSS
Ink set of tools for quick development of web interfaces
JBST Theme framework that can be used as a standalone website builder or to create WordPress themes
KNACSS Minimalist, responsive and extensible style sheet to kick-start your HTML / CSS projects
Kube Minimalistic CSS-framework for developers and designers
Metro UI CSS Set of styles to create a site with an interface similar to Windows 8
Pre CSS framework
prelude Lightweight, responsive and extensible front-end framework
Schema Light, responsive, and lean frontend UI framework
Semantic UI UI component framework based around useful principles from natural language
UIkit Lightweight and modular front-end framework for developing web interfaces
ngBoilerplate An opinionated kickstarter for AngularJS projects
less-rail Less.js for Rails
Wee Lightweight front-end framework for logically building complex, responsive web projects

Grid Systems

Flexible Grid System
Golden Grid System
LESS Zen Grid
Responsive Boilerplate

Mixin Libraries

3L Mixins library
animate Library of CSS3 keyframe animations
Clearless Collection of mixins
Css3LessPlease Conversion of to LESS mixins
CssEffects Collection of CSS style effects
Cssowl Mixin library
cube.less Animated 3D cube using only CSS
est Mixin library
Hexagon Generate CSS hexagons with custom size and color
homeless Mixin library
LESS Elements Set of mixins
LESS Hat Mixins library
lessley A jasmine like testing suite written in pure less
LESS-bidi Set of mixins for creating bi-directional styling
LESS-Mix Mixins library
media-query-to-type Media Queries to Media Types with Less
More-Colors.less Variables for easier color manipulation while you design in the browser
more-less Mixin lib supporting LESS 1.7
More.less Mixins, animations, shapes and more
more-or-less for-loops and other functions + css3 mixins
normalize.less Modularized famous normalize.css using less
Oban Collection of mixins
Preboot Collection of variables and mixins. The precursor to Bootstrap
prelude-mixins Collection of mixins
Shape.LESS Collection of mixins for various shapes
tRRtoolbelt.less Mixins and functions for common actions

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Thanks for thinking about contributing. Please read the contributing instructions carefully to avoid wasted work.

Install These Tools

make sure the paths are setup. If you start your favourite command line and type node -v you should see the node compiler. If you run phantomjs -v you should see the phantomjs version number.

  • clone your less.js repository locally
  • navigate to your local less.js repository and run npm install this installs less' npm dependencies.
  • If you haven't used grunt before, run npm install grunt-cli -g - this allows you to use the "grunt" command anywhere


If you go to the root of the less repository you should be able to do grunt test - this should run all the tests. For the browser specific only, run grunt browsertest If you want to try out the current version of less against a file, from here do node bin/lessc path/to/file.less

To debug the browser tests, run grunt browsertest-server then go to http://localhost:8088/tmp/browser/ to see the test runner pages.

Optional: To get the current version of the less compiler do npm -g install less - npm is the node package manager and "-g" installs it to be available globally.

You should now be able to do lessc file.less and if there is an appropriate file.less then it will be compiled and output to the stdout. You can then compare it to running locally (node bin/lessc file.less).

Other grunt commands

  • grunt benchmark - run our benchmark tests to get some numbers on performance
  • grunt stable to create a new release
  • grunt readme to generate a new in the root directory (after each release)

How to Run Less in Other Environments

If you look in the libs folder you will see less, less-node, less-browser. The less folder is pure javascript with no environment specifics. if you require less/libs/less, you get a function that takes an environment object and an array of file managers. The file managers are the same file managers that can also be written as a plugin.

var createLess = require("less/libs/less"),
    myLess = createLess(environment, [myFileManager]);

The environment api is specified in less/libs/less/environment/environment-api.js and the file manager api is specified in less/libs/less/environment/file-manager-api.js.

For file managers we highly recommend setting the prototype as a new AbstractFileManager - this allows you to override what is needed and allows us to implement new functions without breaking existing file managers. For an example of file managers, see the 2 node implementations, the browser implementation or the npm import plugin implementation.


If you look at, This is an overview diagram of how less works. Warning! It needs updating with v2 changes.