Universal Components & Deterministic Rendering

With the advent of React Native, we became able to develop user interfaces for multiple platforms using the same technology. What would it take to write the same application and have it working in all platforms? Why are we not doing this everywhere already? We tried over the past years and we succeeded, but we also learned some fundamental lessons about user interfaces that tend to be missed when doing web development. I present our learnings here, together with a proposal for the next steps that naturally follow