Ian Hixie provided an insightful quiz to show that there’s more to writing proper markup than running it through a validator. The answers are educational and identify several common misconceptions about which tags are required and how tags should be used. Proper and accessible pages take work beyond what a validator can easily check.
I was happy to get 4 out of 4 in a minute. The errors seemed obvious to me, almost as bad as someone using the <a> tag just to create underlined headings. Based on the results, there’s a lot of educational work to be done to teach people how to create proper markup. The quiz also makes it obvious why automatic conversions from a non-markup format (such as Word .DOC or Adobe Acrobat .PDF) to a markup language generate such poor results.