A browser app is different from a phone app in terms of screen-design, ui and workflow. If you develop a new concept a browser app can be used to prove it, because it is in general easier to create: More space and more widgets available. On the other hand, if your final goal is phone app, it may be misleading. I would start with the phone app.
Thanks for your response. In fact I spend a fair amount of time doing the app design using a mock-up and now the developer that is doing the implementation wants first to do this as a regular website (using Mendix).
I'm afraid this will cost a fair amount of additional (re-design) time and money and I was wondering if this was really necessary. Your answer clearly indicated that this is not the case.