webkit != safari. You might "just" need to build a UI, but if you want it to be cross-platform, that brings you back to the question of what GUI toolkit to use, or you have to handle the mess of different APIs for different platforms yourself.

You’re really confused. We’re not talking about a browser, just an app platform. You can build WebKit on every major platform. If you’re talking about including a UI kit, there’s unlimited of those already written in HTML/CSS/JS. Once WebKit is running there’s no “different APIs”... that’s the whole point.

As I understand it webkit is just the rendering engine. You would still need to build the chrome around it, create and manage windows, etc.