CheerpX is interesting:
We are talking about an always available, zero-cost, virtual machine with guaranteed data privacy .
CheerpX is able to run compiled x86 code (so any source language) in the browser with an HTTP based read-only filesystem delivering the OS (Debian in this case), and local writeable storage in the browser (I think). Uses WebAssembly in the browser.
Some ideas for a Safe hosted OS perhaps?