OSDev.org

web oses are a multifaceted chicken and egg issue

It boils down to

How can you write a web os in a web os?
How can you write the backend software that runs all of the frontend?
How can you except something to work that is tied to the internet constantly and consistantly?
Can a web os ever be self hosting?
And can you acutally get your information out of the os?

I'm pretty sure you could write a C compiler in JavaScript, compile a new web OS, copy it to a disk, then run it. Any Turing-complete (up to memory constraints) system that can copy data to a floppy or USB or CD should be self-host-able, which is pretty easy to satisfy for a system. The same properties let you do the rest of the stuff on your list provided you have an Internet connection whilst writing/compiling/copying your code.

I also think this is taking the web OS idea too literally - see my first post on this thread.