CSS-in-JS, introduced by Facebook engineer Christopher Chedeau in 2014, has gained popularity for its ability to locally scope styles using a modular JavaScript approach, thus preventing unintentional global style overrides that can disrupt application design. However, it typically executes styles at runtime, increasing the JavaScript bundle size. The library "treat" addresses this by executing styles at build time and injecting them into the application header, reducing runtime overhead and keeping the JavaScript bundle lean. It supports type safety with TypeScript, legacy browsers like IE 11, and is framework-agnostic, usable with Angular, Vue, and React, though it requires webpack for bundling. Treat facilitates the creation of static CSS files, offers a dedicated React API, and supports server-side rendering and bundle splitting, providing a solution to CSS-in-JS runtime challenges while ensuring compatibility with older browsers.