You can use React Native for Web [1].

[1] https://github.com/necolas/react-native-web

There are several React Native web players. One is used in their documentation site.

Is that really comparable though? You use that when you want to demo React Native on the web. This means you can actually code for both platforms.

Right, AFAIK none of the available players are advertised as official ways to run apps in production.

Well, in case of the web I meant react

And that's the thing. You need to reimplement your UI code separately for react and react-native. react-xp abstracts all that away.

That's something different though. It looks similar, but there are differences - and this library is solving them.