Designers who design for the web should know HTML, CSS, and Javascript pretty well. The best ones know them throughout. Design isn't art -- it's a technical profession and one that requires the designer to know their medium. If you do not then I would say you're taking the piss right out of it.
Being the person who has to "code" a designers' work is a career built on frustration. The web wasn't built in photoshop. It was built by nerds and engineers. It has come a long way to allow people to make their HTML documents more expressive (to the point that they've become full application UI's...). But that doesn't escape the fact that this is a technical medium with specific technologies that drive those expressions.
You have to know this stuff if you want to be good, IMO.