The PostCSS plugin you've been waiting for: a PostCSS custom parser plugin for integrating the popular Less.js CSS pre-processor into your PostCSS workflow! It integrates the entire Less engine, evaluates your .less, and exports a PostCSS AST that you can use to attach multiple subsequent PostCSS plugins.

Instead of trying to assemble a hodge-podge collection of PostCSS plugins that "emulate" a pre-processor, use a pre-processor!

THAT'S RIGHT. This plugin doesn't give you "Less-like" functionality, or "Less-like" parsing. It gives you the full awesomeness of Less, with the flexibility of PostCSS. Basically, you can throw your "pre-processor-y" plugins away now.

• Because this uses the Less.js parser and not the default PostCSS processor, some parsing will be different. PostCSS accepts "broken" CSS, whereas Less doubles as a de facto CSS linter, and will return errors if your Less / CSS is poorly structured.
• PostCSS will also sometimes "fix" CSS that uses property hacks, which Less preserves as the property name, or will remove comments from within values, which are also kept in the value by Less.
• The less() plugin needs to be the first plugin called.
• Less.js does not save "raws" when parsing. It also only preserves the start line / column of your source, which is still fine for Source Maps.

A plugin like less-plugin-autoprefix parses your CSS twice. First, Less.js parsers it and outputs CSS as a string. Then, PostCSS parses the string to make an AST. The postcss-less-parser plugin directly converts the Less AST to a PostCSS AST without re-parsing.

I'm not sure if that would have a major performance difference or not. Someone else can do the benchmarks, my brain is full.

Follow these simple steps to use postcss-less-parser.

Add postcss-less-parser to your build tool:

npm install postcss-less-parser --save-dev

var less = require('postcss-less-parser');
less({ /* Less.js options */ }).process(YOUR_CSS, { parser: less.parser });

Load postcss-less-parser as a PostCSS plugin:

var less = require('postcss-less-parser');
postcss([
    less({ /* Less.js options */ })
]).process(YOUR_CSS, { parser: less.parser }).then(function (result) {
	// do something with result.css
});

Add [Gulp PostCSS] to your build tool:

npm install gulp-postcss --save-dev

Enable postcss-less-parser within your Gulpfile:

var postcss = require('gulp-postcss');

gulp.task('less', function () {
    return gulp.src('./css/src/style.less').pipe(
        postcss([
            require('postcss-less-parser')({ /* Less.js options */ })
        ])
    ).pipe(
        gulp.dest('./css')
    );
});

Add [Grunt PostCSS] to your build tool:

npm install postcss-less-parser --save-dev

Enable postcss-less-parser within your Gruntfile:

grunt.loadNpmTasks('grunt-postcss');

grunt.initConfig({
	postcss: {
		options: {
			parser: require('postcss-less-parser'),
			processors: [
				require('postcss-less-parser')({ /* Less.js options */ })
			]
		},
		dist: {
			src: 'css/*.css'
		}
	}
});