One realization I had when designing the API of React Native is that if you want to be able to share any component you need to have the lower level primitives to be shared. So I went on making them: View, Text and Image. I also realized that the same layout engine was required so implemented a shared layout which ended up being flexbox.

What amazed me the most during this journey is that most platforms have actually very similar low level building blocks but unfortunately slightly incompatible in many ways.

The 85% number is mind blowing, it turns out that those few building blocks are actually a large part of what it takes to build a high quality mobile app.