Macs ignore hinting when rendering antialiased. Try the prehinted version (probably 72 or 144 DPI). It also works best with dark on light on the Mac due to gamma.
Regarding PNGs, the samples are rendered with hRGB subpixel rendering. I don't think Macs have ever used anything else, but that may be a possibility. Hopefully its not just browser resizing.
The fact that light-on-dark text looks terrible on Macs isn't due to gamma—it's due to the fact that subpixel rendering for light-on-dark text is broken on Macs.
Generally I like Mac OS X's font rendering much better than Windows's, but this is one aspect that drove me up the wall when I was redesigning my personal website. (I wrote a bit about the issues I had with this bug in this article: http://kronopath.net/blog/dawn-of-a-new-day/. Search for "The bug".)
(Yes, I know why this is the case technically, but I don't need to get into that to make my point.)