There are lots of things we need to specify for XUL items, like image-align and image-width for titledbuttons. We may want these new properties to be errors/hidden in HTML, but we're piling hacks upon hacks right now to work around not having any extensibility in the style system. Things like action sheets will also require this sort of hook.
What specifically are you asking for? CSS is not a user/author extensible language in its property set. Things like styling images in titledbuttons can be done very easily using existing CSS properties and pseudo elements (which *are* user extensible). Action sheets requires formal support from the CSS parser, so it is not a generic extensibility issue.
images on titledbuttons could indeed be done using pseudoelements.
Closing this out. Talk to me (or Pierre) about adding specific extensions to CSS if you *really* need them. Also feel free to ask how existing code and properties can be leveraged by using pseudo elements.