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What I did hear a few years a go is that some Yahoo editors use Macs. That was back when submission was free - it may well be that at this point money talks. I'm not sure it's critical to test Mac compatibility, but being able to navigate without JS is IMHO something that is too chancy to neglect.
You may get a better idea by looking at some of the recent additions, in the old days there was quite a bit of fear and trepidation involved in the free submissions, and a common approach was to submit lean'n'mean with any bells and whistles added only after inclusion.
(Probably still a good policy for heavy affiliate sites.....)