Anyone who has been developing for any length of time is painfully aware of box model differences between browsers and even versions for different operating systems. Testing for these differences has been difficult or costly, and the differences are usually minor unless you screwed up your CSS.
Adobe now offers “BrowserLab” which allows Designers/Developers to compare snapshots of their sites in several different versions of browsers in different operating systems. For example, you can compare your site in MSIE v7 vs MSIE v8, or MSIE 8 vs Firefox, or Firefox on Windows vs OSX. The tool even allows you to overlay two of the browsers and use a slider to view the differences, called onion skinning. The tool is free for now, but will likely be charged for in the future.
I was able to do this on a site I completed recently and saw that MSIE used less space in the header than did firefox by about 5 pixels. Firfox on OSX was narrower for the page than was windows. But overall my site degraded nicely in all browsers and operating systems. Can’t ask from much more than that where not pixel critical. If there had been major differences I would have seen the page differences and made adjustments to code. Below is a sample of the onion skin effect.
