[jitl]

Styletron create “atomic” (single property) CSS classes as your application runs in production because it has no macro, compiler, or bundled plugin. “Build-time” means the CSS is somehow extracted from the JS files and inserted into HTML or static CSS files during the bundling process, so that the JS to compute the styles never runs on the client/production. There’s only whatever code is needed to add the generated class names to the DOM nodes created by React (etc) when the app runs.