- standards are really detailed at this point and a large reason why the three remaining browser engines (chromium, safari, and firefox) largely do exactly the same things.
- There are a lot of open source components. It's not necessary to start from scratch on things like wasm and javascript interpreters for example. There are some nice low level graphics libraries out there as well. And of course things like Rust are now pretty mature and there's a lot of rust code out there that does stuff that a browser would need.
That being said, it's one hell of a hobby project to take on and I don't see much economical value in an independent implementation of something provided by free by three independent browsers already; two of which are open source.
Which begs the question: why?!? Is there qualitative argument here of doing the exact same thing but somehow better?