Isn't NN still being developed from Mozilla source? If so, it should have the same rendering ability as Firefox with some added/moved browsing features to make it look different. Also look for some FF extensions to be integrated into NN8.
... just noticed that from the release notes you can also use the Trident (IE's) rendering engine, as liorean said. So the answer is: NN8 can support either the same level of CSS as firefox, or it can have lesser CSS support with the Trident engine enabled.
I'll download it when I convince myself to waste more space with yet another browser I won't use.
Edit: OK, I got it, and I really don't like the UI. I'll admit it's functional, but the orange and light blue blows and IMO it's cluttered. There are no alternate themes for it yet it seems. I find it extremely funny that clearing the history gets its own button and I don't think a popup blocker is important enough for its own button either. As I expected, some FF extensions are built into it. It seems slow, as well. I can't find any option for enabling gecko, so perhaps it's just trident? If so, I expect this browser to flop as even developers won't want anything to do with it. IMO stick with FF or Opera, NN8 seems to be a slow and poorly designed browser based off the Mozilla platform with its share of bugs (I know it's a beta) and a deprecated rendering engine if in fact you can only use Trident.