Really cool article (and magazine more generally) for thinking about things you can do with css and javascript (and getting advice/code). The options include: an analog clock; dynamic, layered index cards; creating perceived depth and 3d ribbons; newspaper layouts; navigation bars; and more.
I have messed around with this, and I think I came near graduating the free class that they offer. It was useful, but I didn't find that it got me too far into javascript. I did find that I liked the points and badges. Did you see the pay option alternative site (I think linked off Code Academy somewhere ... or it comes up when you search Google for Code Academy) where you can learn various coding languages as you play games? One of them teaches you CSS or Javascript as you fight zombies. If you didn't have to pay ... I would totally get into that. :)