StyleX is Meta's CSS-in-JS framework that compiles style definitions at build time into atomic CSS classes. The core is a Babel plugin that transforms `stylex.create()`, `stylex.defineVars()`, `stylex.keyframes()`, and other API calls into optimized JavaScript objects with deterministic class names and CSS rules. A small runtime injects the generated CSS into the DOM with priority-based ordering. The monorepo includes a comprehensive set of tools: an ESLint plugin for enforcing best practices, a CLI for pre-compilation, bundler plugins (Vite, Webpack, Rollup, esbuild, Rspack, PostCSS), a Chrome DevTools extension for debugging, and a documentation site built with StyleX itself. The architecture is modular, with separate packages for the runtime, compiler, and integrations, all sharing common utilities. The framework supports theming via CSS custom properties (`defineVars`/`createTheme`), compile-time constants (`defineConsts`), contextual selectors (`stylex.when`), and experimental nested token APIs. It is production-ready, used at Meta, and actively maintained with frequent releases.