A few years ago, I built a "front page" for my site, using CSS to implement a flexible column system (instead of fixed width). Works fine under IIS 7, but when I upgraded my web server to 2008 R2, the page content displays but no CSS activity is evident.
I brought in a couple pages from a simpler site that works in a different folder, and that works fine, proving that CSS can behave just fine.
I have compared the two pages, and the only differences are in the accompanying CSS code.