The Web is a very wide area and it depends on what kind of work you want to do.
I would recommend to learn at least one server side language and framework like Ruby on Rails or Python/Django. At least in smaller companies developers are expected to be able to do the complete work necessary to get a site online.
You should have good knowledge of HTML, CSS, XML. Version Control tools like git. Test driven development. Knowledge of SQL and database design is important too, since at one point you will have to store user data.
Another interesting kind of knowledge is interacting with the big players and their web APIs. You can explore this easy with Google Documents or Amazon which implement a lot lot of functionality that you could access from your server application and integrate in your website. (There are much more)
Knowledge of the Unix commandline and some of the tools used there is a big help.