The fundamental problem is that if you do end up reimplementing a slightly better DOM yourself, you end up with (e.g.) 20 MB of browser and rendering code. This will have to be downloaded by the client. If this were a onetime thing (perhaps some kind of "install"), it wouldn't be a problem.

But browser caches aren't generally up to the task of making it a onetime thing because:

-They're usually pretty small.

-They'll evict pretty frequently.

-They're limited to same-origin, even if the content cached is exactly the same. (For security reasons, yes, but there's no way to opt out.)

If caches were fixed, I think we would see a much healthier web ecosystem. Incidentally, it would make the browser vendors themselves fairly obsolete, which is probably why it hasn't happened yet.

I think billybolton is right that HTML/CSS/JS/the DOM all kind of suck, and that they can't feasibly be replaced is problematic. The heroic modern struggle of the web ecosystem is an attempt to encapsulate and work around the suckiness.