MS making their browser work on other OS's makes about as much sense as MS making their file system explorer app work on other OS's.
I would rather parallel IE to various document viewers (.doc viewer, for example) which Microsoft provides free of charge. And which, sadly, also aren't available for other platforms.
And the idea (by extension) that any app that works with standards-based content should be cross-platform is barmy.
They're trying to sell Windows licenses. They have no reason to sweat to make a hardware accelerated IE work on OSX, I'm baffled that people think it'd make sense for them.
So by releasing it on Windows they can (a) gain marketshare so it's taken more seriously, and (b) make sure developers who use Windows will test with it.
As for why they didn't just use Gecko (Firefox's rendering engine), Apple also wants to be certain that websites look great on the iPhone/iPad. And my understanding is that Gecko is an absolute monster to get working well on mobile devices. Plus by having their own rendering engine they get to be certain that there'll be no problems adding features needed for touch-based apps to it.
>>So by releasing it on Windows they can (a) gain marketshare so it's taken more seriously, and (b) make sure developers who use Windows will test with it.
Exactly, and in my opinion, Microsoft should follow this logic for the same reasons.
No-one is going to say "I'm not setting up a Windows machine just to test a niche browser like IE. Screw that".
Tasman supported — in it's own buggy way — properties like display:inline-table that Trident didn't support until IE8.