Autoprefixer is a tool to parse CSS and add vendor prefixes to CSS rules using values from the Can I Use. This gem provides Ruby and Ruby on Rails integration with this JavaScript tool.

The best way to use Autoprefixer is with webpack or Gulp.

Autoprefixer Rails doesn’t support this Autoprefixer’s features:

• Browsers in package.json.
• Custom browsers usage statistics.

Windows users should install node.js. Autoprefixer Rails doesn’t work with old JScript in Windows.

Add the autoprefixer-rails gem to your Gemfile:

gem "autoprefixer-rails"

Clear your cache:

rake tmp:clear

Write your CSS (Sass, Stylus, LESS) rules without vendor prefixes and Autoprefixer will apply prefixes for you. For example in app/assets/stylesheet/foobar.sass:

:fullscreen a
  display: flex

Autoprefixer uses Can I Use database with browser statistics and properties support to add vendor prefixes automatically using the Asset Pipeline:

:-webkit-full-screen a {
    display: -webkit-box;
    display: -webkit-flex;
    display: flex
}
:-moz-full-screen a {
    display: flex
}
:-ms-fullscreen a {
    display: -ms-flexbox;
    display: flex
}
:fullscreen a {
    display: -webkit-box;
    display: -webkit-flex;
    display: -ms-flexbox;
    display: flex
}

If you need to specify browsers for your Rails project, you can save them to

• browserslist and place it under app/assets/stylesheets/ or any of its ancestor directories

  > 1%
  last 2 versions
  IE > 8 # comment
  

• Or config/autoprefixer.yml

  flexbox: no-2009
  browsers:
    - "> 1%"
    - "last 2 versions"
    - "IE > 8"

See Browserslist docs for config format. But > 5% in US query is not supported in Rails, because of ExecJS limitations. You should migrate to webpack or Gulp if you want it.

Note: you have to clear cache (rake tmp:clear) for the configuration to take effect.

You can get what properties will be changed using a Rake task:

rake autoprefixer:info

To disable Autoprefixer just remove postprocessor:

AutoprefixerRails.uninstall(Rails.application.assets)

If you use Sinatra or other non-Rails frameworks with Sprockets, just connect your Sprockets environment with Autoprefixer and write CSS in the usual way:

assets = Sprockets::Environment.new do |env|
  # Your assets settings
end

require "autoprefixer-rails"
AutoprefixerRails.install(assets)

If you need to call Autoprefixer from plain Ruby code, it’s very easy:

require "autoprefixer-rails"
prefixed = AutoprefixerRails.process(css, from: 'main.css').css

You can specify browsers by browsers option:

AutoprefixerRails.process(css, from: 'a.css', browsers: ['> 1%', 'ie 10']).css

You should consider using Gulp instead of Compass binary, because it has better Autoprefixer integration and many other awesome plugins.

But if you can’t move from Compass binary right now, there’s a hack to run Autoprefixer after compass compile.

Install autoprefixer-rails gem:

gem install autoprefixer-rails

and add post-compile hook to config.rb:

require 'autoprefixer-rails'

on_stylesheet_saved do |file|
  css = File.read(file)
  map = file + '.map'

  if File.exists? map
    result = AutoprefixerRails.process(css,
      from: file,
      to:   file,
      map:  { prev: File.read(map), inline: false })
    File.open(file, 'w') { |io| io << result.css }
    File.open(map,  'w') { |io| io << result.map }
  else
    File.open(file, 'w') { |io| io << AutoprefixerRails.process(css) }
  end
end

By default, Autoprefixer will change CSS indentation to create nice visual cascade of prefixes.

a {
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box
}

You can disable it by cascade: false in config/autoprefixer.yml or in process() options.

Autoprefixer will generate source map, if you set map option to true in process method.

You must set input and output CSS files paths (by from and to options) to generate correct map.

result = AutoprefixerRails.process(css,
    map:   true,
    from: 'main.css',
    to:   'main.out.css')

Autoprefixer can also modify previous source map (for example, from Sass compilation). Just set original source map content (as string) to map option:

result = AutoprefixerRails.process(css, {
    map:   File.read('main.sass.css.map'),
    from: 'main.sass.css',
    to:   'main.min.css')

result.map #=> Source map from main.sass to main.min.css

See all options in PostCSS docs. AutoprefixerRails will convert Ruby style to JS style, so you can use map: { sources_content: false } instead of camelcase sourcesContent.