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...
I'm not really complaining; just harking back to a shitty time that is, on some commercial sites, threatening to return. Especially as Apple finally allows Chrome on iOS, eroding the main bastion against it: Safari.