LESS is a dynamic stylesheet language that extends CSS by adding features like variables, mixins, operations, and functions. It allows values to be specified once and re-used throughout stylesheets. LESS runs on both server-side (with Node.js and Rhino) and client-side (in modern browsers). While similar to Sass, LESS was designed to have a syntax identical to CSS so that existing CSS code can be used as valid LESS code. Both Sass and LESS allow writing CSS in a programming-like way, but Sass removes curly braces from its syntax while LESS aims to be as close to CSS as possible.