I'm currently in the middle of a large and poorly specified project, so I feel your pain on this. However, it seems like there has to be a way to properly communicate what's needed in a small project without requiring a formal document that may take a significant amount of time for the customer to write. It also seems like these documents never include what I need on the first try, and I end up having to go back and ask for lots of clarification anyway. Sadly, most of the people who are asked to write the specs for this stuff know absolutely nothing about issues of web UI like confirmation pages and error handling, and even less about tech issues like which templating tool will be used.
Maybe the answer to this problem lies in one of the lightweight methodologies like Extreme Programming. I don't have any hands-on experience with XP (yet!), so it's hard to tell.