Lets say for example that you had an html document and a css document, and you wanted to know what selectors matched a given hpricot node, you could do something like this:

class DH < CSS::SAC::DocumentHandler
  def initialize(node)
    @node = node
  end
  def start_selector(selectors)
    selectors.each { |sel| p sel if sel =~ @node }
  end
end
parser = CSS::SAC::Parser.new(DH.new(some_hpricot_node))
parser.parse(File.read(ARGV[0]))

I think I will probably release what I have in trunk soon. I will probably add a new document handler for 0.2.0 that collects all selectors and properties together, and give a nice interface for finding styles that apply to an hpricot node.