I'm the run of the mill wantrepreneur with a vision and a first draft business plan. (Hypothetically speaking) I get the funds raised to get the prototype built. What context should I present to you (the programmer) for what I want built? Eg. Powerpoint? Sketches on napkins?
Learn HTML and CSS and use that. If you expect to work with somebody technical, build something technical, and be a technology entrepreneur, get technical.