I have had prospective clients do it from non-interactive graphic mock-ups -- just pictures! They assumed that was the hard part and just "wiring up the buttons" would be a short simple task. Those were frustrating discussions.
But if you want reasonable portability of the interface across different devices, and scale, and connection quality there's more to do.
Even just getting an interface that responds cleanly to resizing can be trickier than it looks because what is important changes as aspect and scale change. How you present things may categorically change.
And this doesn't even start on talking about how to get the backend to where it matches the implied functionality of the front end.