Sometimes pesky web design problems can be the result of the tiniest little errors that occur at other points on a page. The other day while I was tearing my hair out trying to find out, I came across a fantastic plugin for Firefox that does webpage validation. It's based on the W3C's Tidy libraries, and the UI is truly top-notch. It exists as a small icon in the lower-right-hand-corner of the window, but mostly of the functionality is seamlessly embedded in the 'View Source' window. Very cool.