Ironically I'd find more value in "Windows XP in a browser" than in a custom OS. if it has filesystem access and can handle most of the underlying API. Reason being that I run Linux, so if I need to run a Windows program (which I occasionally do: I'm into video game music, and a lot of the tooling stack for manipulating files is dinky Windows programs developed by Some Guy in 2013 and never really maintained), I have the following choices:

- dualboot (not doing that again, Windows 10 loved to eat my bootloader over and over and would get stuck in update loops)

- wine, which requires a lot of configuration and has weird bugs, but is really good for more heavyweight apps (foobar2000 is still king)

- virtualization, which also requires a lot of configuration. need to perform a full OS install, etc. I haven't found a way to easily spin up a virtualized windows box (I run Manjaro = Arch, let me know if you have an easy way. A while back I gave it a couple hours and couldn't figure it out, so I gave up.).

A Windows-in-browser that runs "well enough" and can access my local filesystem would let me just run the damn app, do the thing I want to do, and then call it a day. Of course, I'm sure there's lots of details I haven't thought through here. But it feels like a potentially legitimate use case.