Microsoft has outlined the CSS extensions whose support has changed in IE 8. These extensions, all prefixed with "-ms-" can be divided into two groups. The first group is to support the work in progress, CSS 3. Since these are still subject to change, Microsoft wants to distinguish between IE's current implementation and whatever form the final specification may take.
The second group of extensions replace the non-standard extensions that existed in previous versions of IE. As per the CSS 2.1 specification, these properties now have vendor-specific prefixes.
While some applaud Microsoft's commitment to web standards, there are a lot of mixed feelings over this change. Many echo's dman's concern:
Yeah. More broken websites in the name of "standards!" So essentially, I need to update all of my websites from overflow-y to -ms-overflow-y. But I can't remove the overflow-y because IE6 and IE7 need that. Then of course when IE8.1 or IE9 (whatever is next) comes around and supports CSS3, I need to revert back to overflow-y because now it will be standardized. But I can't remove -ms-overflow-y because IE8 needs it. Essentially, this change guarantees that websites will NOT be standards compliant for years! If you just leave it at overflow-y maybe people will complain until CSS3 is standardized, but at least after that the madness ends rather than just continuing for years to come.