I honestly don't know! What I know is that there is an intersection of features that work with polyfills and tweaks in all shells. That intersection has a lot of the spec, but also has a lot that is not specified anywhere.

I started to build something here 12 years ago: https://github.com/Mosai/workshop. Then here 2 years ago: https://github.com/alganet/coral

Now I'm attacking it from another angle, trying to document what I learned methodically and share the tools I made for me with everyone else.