They purposely decided not to use CSS for react native, by design.

https://facebook.github.io/react-native/docs/style.html#cont...

Quick example why its not supported: Lets say you are building an instagram style app. You need to render an image from network and won't tell the app its vertical height(Just like you would do in a webapp)

The app has to judge the image height after the image is finished loading from the network. If the second image loads faster than the first image, then UI has to push the second image further below to make space for the first image. This will make the app look jittery.