UnoCSS is an instant on-demand Atomic CSS engine that reimagines how developers approach utility-first CSS. Unlike traditional frameworks that ship with predefined utility classes, UnoCSS generates CSS utilities on-demand based on your actual usage, resulting in highly optimized and performant stylesheets.
Core Philosophy
UnoCSS operates on a revolutionary principle: no parsing, no AST, no scanning. This instant approach makes it 5x faster than Windi CSS or Tailwind CSS JIT, while maintaining a tiny footprint of just ~6kb min+brotli with zero dependencies and browser compatibility.
Architecture Overview
At its heart, UnoCSS features a modular architecture built around three core components:
The Core Engine (@unocss/core) provides the fundamental atomic CSS generation capabilities without any built-in utilities. This minimal core enables complete customization through the , where all functionality is delivered via configurable presets.