Write reftests to compare images in order to deal with floating point tolerance. This includes dealing with things like Monitor Profiles, profiles in JPEG (i.e., JPEGs get changed based on colour profile info). See discussion of colour profiles in Firefox 3 here. It would be useful to be able to compare to PNGs with a reftest, perhaps creating an image diff tool, where pixels outside a floating point tolerance (> 0.n) would somehow be highlighted so you can see what is different. Done.