It's not possible to hack into any network using only a single technology, regardless of what that technology is. Part of being a good hacker is knowing the right tools for the job, and every job is different.
That being said, I will say there are certain "hacking" techniques that can be done just with HTML and CSS such as clickjacking iframes.
CSS and HTML are programming languages which main pupose is for web design. But we can not predict as of now how far these two can go in relation to hacking so we are not closing the doors for possibilities.
ggamble: that's still not hacking into anything. also, there are tons of webapplications 'out there' that won't allow you to see the source code, so the "anything is possible" might be a bit exaggerated :)
stultuske, if I can make a web site show or do something that the programmer did not intend for me to see or do ... thats "Hacking". And through that you can attemplt some "Social Hacking". And there is source and generated source. Copying a generated source and modifying it has potential on any website.