Both Less and Sass (Syntactically Awesome Stylesheets) are CSS extension languages—that is, they are languages that extend the CSS vocabulary with the objective of making the development of many, large, and complex stylesheets easier. Although Less and Sass are fundamentally different languages, the general manner in which they extend CSS is the same—both rely on a preprocessor. As you produce your build, the preprocessor is run, parsing the Less/Sass script and turning your Less or Sass instructions into plain CSS.

Less is the official Bootstrap 3 build, while Bootstrap 4 has been developed from scratch, and is written entirely in Sass. Both Less and Sass are compiled into CSS to produce bootstrap.css. Along with that, it also produces its minified version—bootstrap.min.css—and map files. It is this CSS file that we will be primarily referencing throughout this book (with the exception of Chapter 3, Building the Layout). Consequently, you will not be required to know Sass in order to follow this book. However, we do recommend that you take a 20-minute introductory course on Sass if you are completely new to the language. Rest assured if you already know CSS; you will not need more time than this. The language's syntax is very close to normal CSS, and its elementary concepts are similar to those contained within any other programming language.