CSS Bundling for Rails

Use Tailwind CSS, Bootstrap, Bulma, PostCSS, or Dart Sass to bundle and process your CSS, then deliver it via the asset pipeline in Rails. This gem provides installers to get you going with the bundler of your choice in a new Rails application, and a convention to use app/assets/builds to hold your bundled output as artifacts that are not checked into source control (the installer adds this directory to .gitignore by default).

You develop using this approach by running the bundler in watch mode in a terminal with yarn build:css --watch (and your Rails server in another, if you're not using something like puma-dev). You can also use ./bin/dev, which will start both the Rails server and the CSS build watcher (along with a JS build watcher, if you're also using jsbundling-rails).

Whenever the bundler detects changes to any of the stylesheet files in your project, it'll bundle app/assets/stylesheets/application.[bundler].css into app/assets/builds/application.css. This build output takes over from the regular asset pipeline default file. So you continue to refer to the build output in your layout using the standard asset pipeline approach with <%= stylesheet_link_tag "application" %>.

When you deploy your application to production, the css:build task attaches to the assets:precompile task to ensure that all your package dependencies from package.json have been installed via yarn, and then runs yarn build:css to process your stylesheet entrypoint, as it would in development. This output is then picked up by the asset pipeline, digested, and copied into public/assets, as any other asset pipeline file.

This also happens in testing where the bundler attaches to the test:prepare task to ensure the stylesheets have been bundled before testing commences. (Note that this currently only applies to rails test:* tasks (like test:all or test:controllers), not "rails test", as that doesn't load test:prepare).

If your test framework does not define a test:prepare Rake task, ensure that your test framework runs css:build to bundle stylesheets before testing commences.

That's it!

You can configure your bundler options in the build:css script in package.json or via the installer-generated tailwind.config.js for Tailwind or postcss.config.js for PostCSS.


You must already have node and yarn installed on your system. You will also need npx version 7.1.0 or later. Then:

  1. Add cssbundling-rails to your Gemfile with gem 'cssbundling-rails'
  2. Run ./bin/bundle install
  3. Run ./bin/rails css:install:[tailwind|bootstrap|bulma|postcss|sass]

Or, in Rails 7+, you can preconfigure your new application to use a specific bundler with rails new myapp --css [tailwind|bootstrap|bulma|postcss|sass].


How do I import relative CSS files with Tailwind?

If you want to use @import statements as part of your Tailwind application.js file, you need to configure Tailwind to use postcss and then postcss-import. But you should also consider simply referring to your other CSS files directly, instead of bundling them all into one big file. It's better for caching, and it's simpler to setup. You can refer to other CSS files by expanding the stylesheet_link_tag in application.html.erb like so: <%= stylesheet_link_tag "application", "other", "styles", "data-turbo-track": "reload" %>.

How do I avoid SassC::SyntaxError exceptions on existing projects?

Some CSS packages use new CSS features that are not supported by the default SassC rails integration that previous versions of Rails used. If you hit such an incompatibility, it'll likely be in the form of a SassC::SyntaxError when running assets:precompile. The fix is to bundle remove sass-rails (or sassc-rails, if you were using that).


CSS Bundling for Rails is released under the MIT License.