Almost all developers have personal web projects they like to plug away at in their spare time. This is another question that can help differentiate the passionate developers from the clock-punchers. It's also a good question to end an interview with, as it's usually easy (and fun) for them to answer.
There is a right answer to this question: all of them. A competent developer should be familiar with testing cross-browser compatibility by using all the major web browsers. Obviously they'll have a primary browser they use for surfing, but their answer to this question might be a good way for you to segue to asking how extensively they test cross-browser issues. Also, if it's some kind of css/html position seeing what toolbars they have installed can be a good metric of their skillset (I personally find the web developer toolbar for firefox to be invaluable)
Some might argue that knowing what acronyms actually stand for is trivial, but there are certain acronyms that a developer should have hard-wired into their head ( HTML or CSS, for example). This is the kind of question that might be better reserved for the phone interview to weed out those who are very unqualified.
I've often felt that the more I learn, the less I feel like I know. Solving one mystery opens up ten others. Having the interviewee tell you their faults can reveal a lot about what they know.
Asking about feature additions is a particularly valuable question - it can reveal if they're skilled in programming in general or if their skillset is pigeonholed into their language of choice.