There isn't really a strict syllabus to work through however there are two layers of important things to know about, the practical skills (such as the languages) and the theoretical skills (such as design theory). The Opera Web Standards Curriculum (you can find it via Google easily) has a good rounded idea of what you need to acknowledge but the general method I would learn if your self-teaching is to just get some good books, keep reading stuff on the web and taking part in places like this and just build up your knowledge as you go. There's only so much you can learn from a book and much of web design is about trial and error (and hard research).
Theoretical items you'll need practical knowledge of will include graphic design (as it's front-end you'll need to be handy with pixels), information architecture (mock-up's, sitemaps, wireframes, etc), design theory (so typography, color, general design, design psychology, etc) and user-experience design (at least the basics of accessibility and usability). While it may seem like an after thought, they will be looking for people who know their craft well. The practical stuff and learning the standards is important, but you really need a well rounded scope of knowledge in the front-end sphere as you'll be expected to be able to make judgements on not just implementation of designs but the usability and visual credibility too. Seems daunting I know, but front-end design is as much a creative subject as a coding gig (in fact, I would say it's less about coding and more about the experience). Hope that helps - this info was based on what I've advised others getting into the field.
BTW if you want book recommendations I've got a good list that'll set you down the right road