Appendix B. CSS Extensions

In the words of its author, Alexis Sellier, Less is a “dynamic stylesheet language, which builds upon CSS’s syntax.” Less is a superset of CSS that extends it with variables, mixins, operations, and nested rules.

It’s great because it can really reduce the amount of CSS you need to write—especially when it comes to CSS3 vendor-specific rules. You can then compile your Less files down to pure CSS.