WebKit and Chromium are hard forks. The former is a fork of KHTML, and the latter is a hard fork of the former. However, in recent years I’ve only seen soft forks, and as for hard forks, I’ve only seen one with Pale Moon, which hard forked Gecko and named it Goanna due to disagreements with the direction the Mozilla Project was taking.
But why wouldn’t any organization make a hard fork, whether of WebKit, Chromium, Firefox, or another browser not based on the three mentioned above?
Possibly more complicated than OSes.
(Depends on the browser, and possibly what counts as the scope of the “OS.” For example, you could be comparing GNU/Linux+wayland+KDE to Netscape 1.0, or you could be comparing modern Firefox to QDOS.)
Remember, a modern web browser contains a Javascript VM, which arguably makes it an OS in and of itself.