Sure it's possible. You have to keep some state to know which things apply to which tags (DIV, SPAN, and so on defining a scope), then looking in your CSS data structure to get the right elements. You just have to apply things like the CSS spec says you should.

I'm not sure HTML::Tree will be low-level enough for you to do that, and you can always right your own parser as a sub-class of HTML::Parser if something else doesn't do what you want.