That’s it. No matter what type of work you’re doing for the web, the end result is always HTML. Server-side languages have to output it. Client-side languages manipulate it. Even rich media, like Flash, must be embedded in it.
Necessity will naturally enable you to learn more and integrate other languages into your skill set, but it’s important to have a good foundation. Once you have that, you’ll start to understand the limitations of what you’re working with, and it’ll be much easier to look for and find solutions, like client-side languages or database-driven sites, that will enable you to do more.
Unless you’re fully competent in the basics, you’re only cheating yourself. Get in on the ground floor, and you’ll work your way up in no time.