It doesn't render CSS, not in that context. The way styling for layout and for colour/text/etc is specified is designed to match a subset of CSS so that developers can easily transition across to the platform, that's all (and for layout it uses an engine called Yoga, which implements CSS' flexbox spec). So in the context of actual component code, it's a layer designed to look like CSS that can generate whatever the backend being used requires to render (so it should be able to produce valid CSS as an output, as in the case of React Native Web)