It's a purely web-based desktop environment sitting at https://linuxontheweb.github.io/. That was the first link on the page. I've already tested it on Chrome, Edge and Firefox. I don't have access to Safari, so you can possibly let me know if there are any issues with it.
> An operating system (OS) is system software that manages computer hardware and software resources, and provides common services for computer programs.
The web browser certainly provides a broad base of hardware and software resources that could be managed to create common services for other apps to run atop.
Don't get too tripped up with limited modes of thinking, with applying only your model of what you know. An OS is an abstract idea: it doesn't have to be limited to a thing running on bare metal. And indeed, given the prevalence of data centers & hyperscalers, I'd wager a very sizable portion of OSes running today are virtualized, are guest OSes running on other host OS.