browser suck, but they are basically the right idea: download untrusted applications over the internet and run them in an sandbox. Note that both ends of the cloud use this model. You consider your code uploaded to a VM in ec2 to be trusted, but amazon don't and they protect their physical hardware from you by running your code in a simulation of a real computer.

Of course, this was never what browsers where conciously designed for, so we ended up with a whole bunch of other cruft, but really, the browser is a hypervisor.