How well do you know XHTML?

My brother told me about this site called Python Challenge which is in turn a variation of the internet riddle Notpron. I was intrigued, and thought it might be fun to create a similar riddle about (X)HTML. Right now, my XHTML Challenge has 20 questions, but I'll add more whenever I think about it. Have fun and see if you can get through the XHTML Challenge.