CLR is not just something that blindly interprets IL instructions. IL
instructions by themselves are useless. Just like any language is useless on
it's own. It's all about runtime and libraries that provide various services
such as forms, graphics, printing, encryption, the whole CAS (code access
security model), garbage collection, web services, remoting, versioning,
cross language interaction, etc etc etc. That, as you might guess, is what
the "Fat" .net framework is about. I'm not going to list many many other
things that .NET provides. Running .Net apps inside a browser VM would suck
performance wise.
It's very far from simple HTML/Browser concept.