This is the first time attempting to validate a site so I apologize if the lingo is off!
I went to The W3C Markup Validation Service to validate my homepage and it's not happy with my superfish generated menu (You can view my site in-development at www.myalgebrabook.com username: a password: a). I've tried adding in another <ul> tag but then the menu doesn't work. So, as a newbie to "site-validation",
Validation is just a guide, so you have to use your judgment. Try fixing one error at a time, as one error can cause a lot of others down the line. There may not be anything wrong with the nav at all.
Thanks so much!