The title doesn't do the linked page justice.

1. It is by Fabrice Bellard, of QEMU, FFMPEG, TCC, BPG, ... fame.

2. It runs Win2000 in QEMU in Linux in JS on your browser. Seriously. Not very quickly, but it works. See https://vfsync.org/vm_list.html ; Also X windows if you prefer.

3. The idea seems to be that you can store encrypted data in the cloud, and use it on any machine capable of running javascript, by booting into a Windows or Linux machine emulated in the browser. Obviously, it is only as secure as the machine you run it on -- but if you can trust that machine, you have an encrypted "fat" client everyone on "thin client" hardware. Dropbox lets you take your data, this lets you take your whole machine.