StyleX is Meta's solution for managing CSS in large-scale applications. It merges the ease of CSS-in-JS with the efficiency of static CSS, generating atomic CSS that avoids style collisions while allowing developers to write expressive, type-safe styles. Open-sourced at the end of 2023, StyleX has become the go-to styling system for Meta's products like Facebook, Instagram, and WhatsApp, as well as for external companies like Figma. The framework addresses historical issues Meta faced with CSS, such as collisions and dependency management, which often led to complex selectors and bloated CSS files. Previous systems, like cx, linked CSS to JavaScript but still struggled with performance due to runtime style injection. StyleX replaced cx by compiling styles at build time, significantly reducing CSS size—by up to 80%—and improving maintainability across diverse codebases. The system ensures styles are predictable and collision-free, allowing engineers to focus on writing code rather than managing styles. StyleX operates with a lightweight API, where `stylex.create()` defines style objects that get converted to atomic CSS classes during the build process. This approach allows for efficient caching and deduplication, optimizing performance as the application grows. The compiler traverses files, extracts style metadata, and compiles it into a static stylesheet. It can handle variable generation, specificity management, and even produce both left-to-right and right-to-left variants of styles. The framework emphasizes expressiveness within constraints. It discourages conflict-prone patterns and supports reusable values across files through functions like `stylex.defineVars()` and `stylex.defineConsts()`. This setup allows developers to share constants and variables without needing to read the contents of imported modules, streamlining the styling process and ensuring consistency across components.