I\'m using some semi-transparent PNGs as background-images on various websites. These are usually something like a 1x1 image with a 30-percent opaque white layer.
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After viewing the sites on both devices (my Mac and my iPod Touch), I think what you're seeing here is a difference in what colors the screens can produce rather than a bug in either rendering engine. There are many factors that can go into this kind of difference, e.g., the contrast in the portable display might not be as great in an effort to conserve power, make the device cheaper, etc.