The main thing is that you'd need a new set of abstractions for security, and then you'd need to implement HTML5 on it anyways to do all the things we can do on a computer now.
Such as what? 99% of websites are a) screens where you enter something into predefined fields, to be stored in a database and/or b) screens that format nicely for display things that you or other people have previously entered. They were doing that in the 1970s. Only the buzzwords change.
If you could start again, would you really end up with HTML5?