Can Selenium or any other automated tool check for the proper positioning of elements on a web page?
For example, when you look at OnStartups.com in IE 6, the main content that is supposed to be to the right of the menu is below it instead and you have to scroll down to read it. Can you query and get the coordinates of the div as they are actually displayed, and can you get the coordinates of the viewport to verify that the top left corner is actually within the viewable area?
I'm afraid the best you could do is select a standard browser and than compare screenshots of other browser with the reference one.
I can't remember the specific one but some tool like Araxis merge was able to compare screenshots in a pretty intelligent way.
I think comparing screenshots is the best thing you can do and I'm afraid there aren't robust tools for that, yet.
If I were you, I would create a script that navigates through every page of the site capturing a picture on each and then I would open a folder with all the results and it would probably take less than a minutes to check 20 or 30 of them. Sounds really dumb, but sometimes the easier alternative ends up being the best...