Internet Explorer lets me down
Most web page makers know that Internet Explorer is a pain in the butt. IE hasn't been updated in a long time, and its support for CSS2 is not very good. Every other major browser such as Opera (my personal preference), Firefox, and KHTML-based Safari (Mac) and Konquerer (Linux/KDE) has better support for standards than IE. Of course, none are perfect, but all are better than IE.
Here's a ridiculous list of IE bugs that some internet ninja has compiled, to back me up. I'm not just hatin'.
I'm not gonna cater to one buggy outdated browser, so instead I found a neat feature (found via this page) in IE that makes it easy for me to put in HTML that only IE users will see without me resorting to server-side tricks. If you're using IE, you're seeing it right now. The rest of you can look at the page source or something.