Converting mockups from "the layout guy" into HTML/CSS was my first non-food-service job in high school, but this was about six years ago, at a small advertising firm that did more print work than web work.

I think the confusion comes from referring to the person who does the mockups as a "web designer." I have yet to meet a person with that title who does not know HTML or CSS. The people who don't get their hands dirty with the coding are usually referred to as just graphic designers.