To anyone who says "you can fork it": If you aren't using the same features as everyone else, how is the situation then different to what we have now?
He is also assuming that because every major browser is using the same engine, everyone will suddenly magically agree with one another on how to do stuff. Chrome and Safari uses different versions of WebKit, and trust me, it won't be long before Opera will use its own version of WebKit as well.
What I'm trying to say is that while it is much easier than it was to create a branch and merge all or some of the modifications back into master, the concept of a fork, where development diverges because different parties want to do different things with the project, which can't easily coexist in the same codebase, is still relevant.
So it's as fragmented as on the desktop.