There's a really useful attribute in CSS called min-height that lets you set the minimum height an element should be displayed as; this is often used to make two boxes appear the same height even if one has less content than the other. Well, Firefox, Opera and Safari support it but Internet Explorer 6 and older don't. Luckily there's a really simple work-around for it, you simply add a defintion to your CSS that browsers other than IE will ignore and set the height to the same as the min-height, e.g.:
[source:css]

With everyone raving about YSlow I used it to prune a new project I was doing. Well, after the results worked just great with Safari and Firefox I was thoroughly confused why it wasn't working correctly in IE. The problem was that my Script.aculo.us routines weren't working, and more specifically I was getting really weird errors when the page was loading, before any of the routines executed. Some of the errors were very vague, e.g. "'Class' is undefined", and "Effect.toggle is undefined".