After eight years of neglect,
the W3C CSS Working Group has resurrected
Behavioral Extensions to CSS.
"Behavioral Extensions provide a way to link to binding technologies, such as XBL, from CSS style sheets. This allows bindings to be selected using the CSS cascade, and thus enables bindings to transparently benefit from the user style sheet mechanism, media selection, and alternate style sheets." This defines a single binding
property and a :bound-element
pseudo-class.