You would probably want to build a browser on top of Chromium, like Opera does (and also what Microsoft is doing on Android), and would want to try differentiat

You would probably want to build a browser on top of Chromium, like Opera does (and also what Microsoft is doing on Android), and would want to try differentiating based on user-facing features. You could also build on top of Gecko, but this doesn’t seem to be as popular. Building on WebKit probably doesn’t make sense because its corporate backer (Apple) mostly only cares about supporting macOS and iOS. Eventually, if you become big enough that you think it’d be better to be independent, you can fork Chromium. This is basically the approach Google took with Chrome: they built on top of WebKit (which itself was originally an Apple fork of KHTML), then forked it eventually.