There's a huge amount of fairly boring work to write a DOM implementation (if you want half the APIs the web expects to work with the right semantics, you can't

There's a huge amount of fairly boring work to write a DOM implementation (if you want half the APIs the web expects to work with the right semantics, you can't use an existing library), and layout is incredibly hard to get right (especially in a web compatible way). I think it's totally doable for a single person to do HTML + CSS2.1, with no scripting support, especially if you're happy to leverage existing libraries. The big difference with most existing browser developers is the majority know a lot of the web platform inside out—but that's mostly a matter of having worked on an implementation for a long time and having to worry about edge-cases and interactions no web developer would ever think of. There's certainly a few who have achieved a huge amount for individual developers—but that's not a trait unique to the browser sphere.