I don't think you're being unrealistic, and I think your quest is worthy. I work with a guy who fits your description: he designs screens and then implements them in HTML and CSS. And it's awesome. When those skills are in a single brain, the result is interfaces that feel native to the web (and not like pretty pictures that were sliced up and made clickable).

But these guys are indeed tough to find. It might be helpful to advertise for two positions -- web designer and front-end developer -- and in each description mention the other skills you expect applicants to have. Most people I think will naturally be stronger in one area than the other, or will just self-identify with one job title or the other for whatever reason. Framing the position in a couple of different ways might help you cast a wider net.