Not only are there non-browser-maker WHATWG stakeholders, there are non-browser-maker formal objectors to the W3C DOM move to CR endorsing the same view as the four browser makers (Bloomberg and Disruptive Innovations; I've never heard of the latter previously, AFAIK.)
Bloomberg is know for its Terminal. It seems they have a program where you can extend their terminal with web applications. Presumably Bloomberg didn't built a renderer themselves but there are custom APIs. So they are something between a browser maker and a UI framework like Electron.
Disruptive Innovations is a firm by Daniel Glazman, formerly of Netscape and of the CSSWG fame. His main product seem to be continuing the Editor part of the original Mozilla Suite based on the Gecko Layout Engine. There seems to be an NVU Editor and rather new an Editor called BlueGriffon which also does ePub and maybe his custom WebBooks format.
