less/less.js

Less.js is the official JavaScript implementation of Less, a stylesheet language that extends CSS (the language used to style web pages). CSS on its own is repetitive, you often write the same color codes, spacing values, or selector patterns dozens of times across a stylesheet. Less adds features that make stylesheets easier to write and maintain. Less lets you define variables (for example, storing your brand color as a name like @primary-color and using it throughout), write reusable chunks called mixins (blocks of styles you can apply to multiple elements), nest selectors inside each other to reflect the structure of your HTML, and perform calculations directly in your styles. When you're ready to deploy, Less compiles your files into standard CSS that any browser can read. Less can run in two ways: in the browser via a JavaScript file that compiles Less in real time, or on the server side using Node.js where it compiles files as part of your build process. The repository is the official Less compiler codebase, it is what powers the less npm package. You would use Less when you are building a website or web application and want to write cleaner, more maintainable CSS without manually repeating values or reorganizing your code every time something changes. It is particularly popular in older projects and those with established CSS conventions; newer projects often use Sass or CSS custom properties for similar benefits.