[selectors4] drag-and-drop pseudo-classes

The CSSWG discussed drag-and-drop pseudo-classes today. The current
proposal is to have three pseudo-classes:

   * One for the element representing the drop target that
     would receive the item if it were dropped.
   * One for all elements representing possible drop targets
     that could receive the item.
   * One for all elements representing drop targets that do
     not accept this type of item.

We'd like comments on
   a) whether this is a correct and useful set of pseudo-elements
   b) what these pseudo-elements should be called, that would best
      (most clearly and succinctly) represent their functionality
      to authors using them

Name sets being considered:

     Set A         Set B       Set C            Set D
----------------------------------------------------------------
  :active-drop    :drop        :current-drop    :active-drop
  :drop           :can-drop    :valid-drop      :valid-drop
  :no-drop        :no-drop     :invalid-drop    :invalid-drop

Thanks~
~fantasai