TL;DR
An open-source design system grown inside Meta, fully customizable and built for both people and AI agents.
Key features
Ships 150+ accessible components and 7 ready-made themes.
Styles are authored with StyleX but invisible to consumers; override with className using Tailwind, CSS modules, or plain CSS.
Open internals: components are built to be composed at any level, and swizzle ejects a component's full source into your project.
API, docs, and CLI are designed together so a person and an AI assistant build the same way.
When to use it
When starting a project quickly while maintaining consistent UI/UX across a large team or organization.
When you want to add accessibility and theming capabilities without replacing your existing styling system.
When looking to integrate your development environment with AI coding assistants for improved efficiency.