I teach the introductory course for a community college web design program, and the very first week we dig into web standards. I give them lots of resources to read and then they discuss it. I found that teaching them why it’s important is fundamental to getting them to be inspired to learn it and use it (which is important when your students come with some experience and bad habits).
I do keep the HTML and CSS portions separate for their major project, which is a 5 to 7 page web site and I do not allow any presentational code for the first half of the class. (But they are taught CSS from the beginning with smaller assignments.) I do this because I want them to know how “beautiful” a properly coded HTML page looks. And because their sites have no design elements in the beginning, if someone does use HTML inappropriately or in a non-semantic manner, boy does it stand out like a sore thumb! They also have to validate their site at each stage which is essential for bug checking and getting rid of deprecated code (we use XHTML 1.0 strict doctype).
I also encourage and teach folks how to dissect web sites and therefor learn from them. The web developer toolbar is invaluable for this purpose, in my opinion. Seeing is believing!
I know that when students leave my class they are usually not only enthusiastic about web standards but they insist on applying them in the rest of the program and in their work environments. :)
To answer the original question, however, I would have to say that teaching students brand new to XHTML and and CSS box model behavior and layout to be the most problematic at this point. It’s very difficult for students to “get” this without hours and hours of trial and error. I’m using new video curriculum from Lynda.com this semester, though, so I’m anxious to see if this helps students understand concepts and application faster.