Only tangentially related of course. But we end up using the Web (webapp in this case) for all sorts of things where “works on my machine” is not even a potential problem.

I am not sure how much web development you have done, but the "works on my machine" problem can and does show up. And not just machines, different browswers and different browser versions. Unless you go very simple, you can get a graphic designer to point out plenty of problems for you.