IE and Quirks Mode (or: always use a doc-type)
I've spent the last few days fighting IE to make my app look right. jQuery accordion was doing all sorts of weird things. Not surprisingly what fixed it was one simple line:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
That was all it took. Even a simple HTML5 doc-type <!DOCTYPE html> would fix this problem. I often start a project with just a simple html tag followed by whatever code I need just to get it running. Later on I run into problems because I forgot this important piece of code.
The specific problem I was having was with jQuery accordion with {fillSpace: true}. In internet explorer it would leave space at the bottom and when changing sliders that space would grow until the accordion was tiny.