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?
Wasn’t Servo Mozillas attempt to solve that before they laid off all Servo staff and gave the project to the Linux foundation?
It was one of the stated goals for Servo itself to be designed like that. But I don’t think anyone at Mozilla expected Servo to take over from Gecko. They were already quite happy that they were able to incorporate Servo’s style engine and URL bar implementation and such into Gecko.