Chalk this one up to strange looking things, Safari now has an option for transparent windows. If you have the 10.3.9 update and the debug menu enabled you can turn on “Use Transparent Window”. I’ve tried this a few times and it only works for pages that don’t have a color or image set for the body tag. It also works in empty tabs. From the comments it seems this has something to do with the Dashboard feature in Tiger (OS X 10.4).