I had thought of something like this a while ago, with the idea of starting at a much lower level and designing the thing to be parallel from the ground up. So that the browser would be stitched together as a bunch of independent units which could run in isolation. Kind of a modular browser, the goal being crashproofing -- eg, one thing failing does not imply everything failing (a la Chrome, which didn't exist when I first thought of this, but is a good enough analogy now). And able to take full advantage of multicores machines -- even as plugins pile in.

I had named the thing "Horizon", but just found it very difficult to get started writing the low-level (eg, what webkit does) stuff. I wanted the thing to be pure haskell principally for the learning experience, but I ended up going another direction instead.