All of them. Don't deliberately shut people out. That's alienating all the others - and there are far more browsers out there in use than you'd think. Provide an alternative. Maybe rethink your design.
Every person is different.
And this also means that you've got to treat your clients as individuals, and we'll look what that entails a little later on. But you need to try your damnedest to be all things to all people. The Internet is rapidly turning into a service economy. They're doing it because in the software world, the product is information - and information wants to be free. Give it away.