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?
keep in mind that the web specifications are incredibly complex. by some counts it’s probably the most complex program most people will ever use. to be clear, that count includes a lot of stuff that has never been implemented. but even so, you would have to read and implement a substantial subset of these to make a web browser. even if you hard fork an existing browser, you have to deal with every new bug and security issue and feature and every new spec the w3c puts out. it’s a huge fucking mess.