Marker interface for all derived values. All methods for any possible style are declared here, which doesn't make complete sense, as, for example, a length can't return a value for asColor(). This is done so that CalculatedStyle can just look up an FSDerivedValue, without casting, and call the appropriate function without a cast to the appropriate subtype. The users of CalculatedStyle have to then make sure they don't make meaningless calls like asColor(CSSName.HEIGHT). DerivedValue and IdentValue, the two implementations of this interface, just throw a RuntimeException if they can't handle the call