[wengzilla]

Not sure this is a legitimate concern though... The experienced CS majors will be able to blaze through HTML/CSS just as they would be able to blaze through Ruby. I also think they'll find themselves sufficiently occupied helping their classmates and/or implementing more challenging projects.

It's fine to start with HTML, but recognize that it's a markup language, not a programming language. Aside from that, you'll be able to cover most of the HTML/CSS within the span of a week (hopefully faster with the CS majors).