Bellard (yes, him) already had a working VM of Windows 2000 in the browser around a decade ago, with no specific "support for all browsers" (whatever that means):
Also, look at 8086tiny, or PCulator/XTulator. These could be ported with ease to EMSCripten and run everywhere, not just in Chrome.
Chrome almost became the new IE and Google, despite it's motto, the new Microsoft. Ok, the desktop? Android rules the smartphone sales in the world, and its online services are used by millions, with far more metadata grabbing than w9x/wxp on its day and with the same propietary OSes.
https://bellard.org/jslinux/vm.html?url=win2k.cfg&mem=192&gr...