Tailwind CSS v4 is a utility-first CSS framework that generates styles from HTML/template content at build time. The core engine is written in TypeScript, with a high-performance Rust-based scanner (`crates/oxide`) for fast file traversal and candidate extraction. It is distributed as a monorepo with multiple integration packages: a PostCSS plugin (`@tailwindcss/postcss`), Vite plugin (`@tailwindcss/vite`), Webpack loader (`@tailwindcss/webpack`), Turbopack integration (`@tailwindcss-turbopack`), CLI (`@tailwindcss/cli`), standalone binary (`@tailwindcss/standalone`), and a browser-ready build (`@tailwindcss/browser`). The upgrade tool (`@tailwindcss/upgrade`) automates migration from v3 to v4, covering CSS directives, template syntax, and JS configs. The architecture emphasizes performance through Rust native bindings, a custom CSS parser, and an advanced AST optimization pipeline. The codebase demonstrates exceptional craftsmanship with extensive test coverage, well-abstracted patterns (state machines, AST walkers, value parsers), and a clean separation between core logic and framework integrations. It is actively maintained with frequent releases and a large community.