OP here. Thanks for the pos/neg/neutral comments everybody. Some remarks: Yes, there are a lot of browsers out there where I can contribute to which would be more efficient. But that would result in just a few browsers instead of a lot of browsers. I'm all for the last one, as this would give people options. This IS a pet project. I never told anyone otherwise. I would start writing it even when I wasn't fed up with…
The only hope, IMO, is to build something much more sane that cannot be abused. Something like Gopher or Gemini. Simple specs that make it possible for a single person or a small team to implement something decently working without them spending their like on it like Middle Age monks spent their life mapping and cataloging stars.
I would start with a feature-limited markup language that's good enough to display text - maybe images if you really insist - and leave all media and interactive stuff to external programs; if your browser ends up being a virtual machine and a universal media player, it is doomed to degenerate into a WWW browser if it ever gets popular.
But of course if one builds something that cannot be abused or monetized easily, the big content creators won't be interested. It will take a lifetime or two to gather a critical mass of users, and only if the WWW overloads abuse their power so much that the average WWW serf actively starts to look for a better place to be, and only if at this time the alternative has more and bigger advantages than drawbacks.