Over at Slash Central, we are actually working on converting Slash -- including Slashdot -- over to XHTML + CSS.
I'm not kidding.
My current task is taking the code we have to massage your comment and journal text into somewhat reasonable HTML, and making it spit out perfectly valid XHTML. And then converting all existing comments to such valid XHTML.
I'm not kidding!
It's actually coming along well, better than I'd have thought. I took 1,000 random Slashdot comments and ran them through my code and, after fixing some bugs found in the process, got the number of XHTML errors from 7,000+ down to 0. And a side-by-side comparison shows them to still look mostly the same. There's a few small/necessary changes, like putting content inside an UL tag in an LI. Which was a HUGE pain.
A lot more to do, but the progress is encouraging.