StyleX: A Styling Library for CSS at Scale

StyleX is Meta's open-source styling system that combines CSS-in-JS ergonomics with static CSS performance through build-time compilation. It generates atomic CSS classes to reduce bundle sizes by 80% while providing type-safe, predictable style authoring. The compiler uses Babel to extract styles, handle specificity automatically, and support features like theming, dynamic styles, and CSS variables. StyleX powers Facebook, Instagram, WhatsApp, and other major applications, enforcing constraints that prevent common CSS pitfalls like specificity wars and styling conflicts. The system includes tooling like ESLint plugins, CLI tools, and bundler integrations, with an active open-source community contributing extensions and improvements.