Also, I'm working on something similar, but from a different perspective. My problem with this is that you have to design your website in Sketch, then turn it into code with a different service potentially waiting hours until you'll see the end result.
So my idea is that I can build an app where you can basically draw the page elements, similarly to how you'd do it in Sketch, and it automagically turns the whole thing into a responsive website right away (no position:absolute hacks involved). I still have a lot to do though... :D