Since you presumably already have some general programing skills, I would suggest learning JavaScript (for front-end programming) and PHP or another common server-side language, along with how to use it in conjunction with a database. Unless you want to become a front-end specialist working mostly on the visual aspects, you don't necessarily need to be a complete HTML/CSS expert, but you definitely want to know all the basics and at least have some idea of what to Google for with regard to the more advanced HTML/CSS concepts.
IMHO, of course.
