If this were a commercial project then I could understand the complaint.. but this is just a small, for-fun project and they have little motivation to put the extra effort into support for all browsers.

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):

https://bellard.org/jslinux/vm.html?url=win2k.cfg&mem=192&gr...