LESS adds functionality that should be in CSS in the first place. For example, LESS gives Web developers variables, functions, operators and other features that CSS doesn't have.
LESS supports mixins, which are properties that can be copied from one ruleset to another. With 1.2.0, Sellier has added conditional execution with its mixins, so that developers can change behavior of the mixin based on the value of parameters passed to it. For instance, developers might change values depending on screen size.
You might be familiar with, Sass, the extension of CSS3. In 1.2.0, Sellier borrows some of the type-checking functions and the mix function from Sass.
LESS is available under the Apache 2.0 license and code is available on GitHub.