UnoCSS is an instant, on-demand atomic CSS engine, not a pre-built UI framework. It offers high customizability through a preset system, with a core design focused on maximum flexibility and performance. UnoCSS generates only the CSS rules you actually use at build time, with no AST parsing, resulting in extremely fast speeds and a minimal footprint (~6kb). It provides first-class support for modern frontend tooling like Vite, Webpack, and Nuxt.