First look at the CSS support in Internet Explorer 8
The trouble is — designers and developers don’t settle for current minimum, we are always into something new. So, by the time a giant vendor locks the feature list, a more flexible and agile competition goes miles away.
In the era of rounded corners/borders and transparency made possible purely with CSS, IE team missed to implement both, or at least in the form that wouldn’t require separate style-sheets files to conform to W3C guidelines.
I checked a few dozens of CSS-based web sites, and I’m not even sure anymore if everything works because developers payed special attention to IE 6 and IE 7, or is it because IE 8 is mature enough to render CSS layouts properly.
What works, what isn’t working
PNGs work without hacking. Rounded corners with CSS not. Text shadow — no. Alpha transparency is still possible only with proprietary filter property, for instance
Naturally, if you want your CSS to validate — this rule has to be extracted in separate file, preferably with conditional comments.