The first reading of this is excellent - I rated you a 10 out of 10 on the cgi resource index. Only one comment - in the sample html for the form variables at the very beginning of the lesson, you use HTML, but your example HTML in the exercises uses XHTML (with trailing slashes). It would be better to be consistent, and use XHTML compliant markup everywhere (and perhaps indicate this as such at the start of the course, along with a pointer to the W3C documentation).