Though not specifically targeted at a Web Application Developer position, you might get some ideas from On Interviewing and Interview Questions.

Since I wrote that article, I've got good value from putting up some code on a whiteboard at the interview with a variety of errors, dubious coding practices, and stylistic discussion points. Apart from testing whether the candidate can find the errors, I've found these sorts of questions doubly useful in watching how they go about finding them (e.g. do they devise "boundary condition" test data and step through the code with it?), and especially in drawing the candidate into a discussion on stylistic issues and sound programming practices in general, and gaining insights into their personality and passion for programming. Passion is probably the main thing I'm looking for.