Ok, I’ve been keeping an eye on this, and with the recent preview 6, there are still some big missing pieces in their CSS3 support:

  • CSS3 gradients
  • Flexible box model
  • Text shadow

Plus, on the JavaScript side, I have yet to be able to get any CSS transforms working, though the way to detect them seems to be:

if (document.body.style.msTransform !== 'undefined')
    alert("IE9!");

In short, it seems to have relative parity with Opera 10 as far as cool-yet-useful CSS3 features. Keep going, Microsoft, I’m totally cheering for you!