According to their website, Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile-first projects on the Web.
You can read more about Bootstrap in Chapter 12, Bootstrap and Sass, of this book. Also, read the Installing Sass for command line usage recipe of Chapter 1, Getting Started with Sass, to learn more about Ruby and RubyGems.
Tip
In this recipe, version 3 of Bootstrap is used to show you how to use Bootstrap with Compass and Sass. In Chapter 12, Bootstrap and Sass, of this book, version 4 is used. Bootstrap 4 was still under development at the time of writing this book. Bootstrap 4 is not backward compatible with Bootstrap 3, but migrate guides will be available. Bootstrap 4 moved from Less to Sass, which means that you do not have to use the bootstrap-sass
gem anymore.