Learning HTML on its own is as good a foundation as it gets in web designing. It at least enables you to undertsnd how a WYSIWYG editor works in terms of output, and enables you to fix things when that output is not as intended or expected.
The natural progression from HTML is to XHTML and this will become more commonplace as time goes on.
The thing about HTML is that if you learn it properly, the same kind of syntax markup can be seen in lots of other lnaguages and learning another one after HTML becomes so much easier. For that reason alone, it's worth spending some time on.
It's always difficult to advise without knowing what it is you like.
If you like the idea of dynamic sites, online trading, e-commerce, databases and whatnot you could do worse than start looking into PHP and MySQL. If the idea of databases sends you running for the hills (as it does with most people) then this is obviously not the path for you.
What do you like about web designing so far, and how do you see yourself moving forward ?
It's important to select a topic that holds your attention if you can. Learning something you don't enjoy becomes a real chore.
Post back with a bit more and get some suggestions and alternatives in return, but my first response is that you should run all you like. If you fall down, get back up and start again. You always know where to come for help if things go wrong.