If you write JavaScript you use npm. It's the best method for publishing and using JavaScript libraries for both server and browser code.

npm is also great for CSS.

Here's a talk from CascadiaFest 2015 talking about methods for using css via npm:

And here's the slides:

This talk provides a quick snapshot of the status of using css on npm. Expect more posts from us providing detailed usage of specific tools, and updates on new approaches.