If this and this don’t get you to cream your UI shorts, you need to put down that jar of AJAX and step away from the screen. Joe Hewitt blogged about Boxely nearly a year ago and its finally looking like it might be nearing some kind of release (if only in AIM Triton).
Style rules, and the triggers that dictate what visual styles are applied when – is 100% based on the state of an element. Specifically the style rules are fully attribute and property driven (e.g when hovered == true for this element apply a procedural color transform). The current complexity of cascading style engines is all due to the need for resolving ambiguity and precedence given the layering of selectors.
With Mac hardware able to boot Windows, it’s only a matter of time before we UI people assimilate the Windows desktop with stuff that actually looks, feel and behaves like the richness we’ve come to take for granted on the Mac.