I am often surprised by how much bad HTML is out there. Especially HTML that does not use Cascading Style Sheets yet. CSS has been around for over 10 years now! It was added to HTML in 1997 and well-supported (at least reasonably) in web browsers by 2000. I think the reason many people never upgraded is that they either learned HTML before 2000, they learned it from sources (friends, coworkers, internet, legacy code) from before CSS, or they learned just enough to get a site online and never bothered learning the "right way".
Legacy sites are not always worth the trouble to update—I know I have plenty of old code still floating around out there—so their flaws are understandable. But some folks are carrying around their bad HTML habits still. If you ask them to code a page they probably know that frames should not be used, so they use a table to layout the page instead. They might use <center> when they want text to center. I see bad code and misused tags all the time. You can "view-source" on many major websites and see bad code by people who should know better.
If that sounds like you or someone you know, keep reading, I will try to help you break your bad habits and start good ones.