When "mouse wheeling" occurs, the implementation must dispatch a
<code>mouseomniwheel</code> event implementing the following interface:

interface MouseOmniWheelEvent : MouseEvent {
   readonly attribute long            wheelDeltaX;
   readonly attribute long            wheelDeltaY;
   readonly attribute long            wheelDeltaZ;
   void               initMouseOmniWheelEventNS(in DOMString namespaceURI,
                                                 in DOMString typeArg,
                                                 in boolean canBubbleArg,
                                                 in boolean cancelableArg,
                                                 in views::AbstractView viewArg,
                                                 in long detailArg,
                                                 in long screenXArg,
                                                 in long screenYArg,
                                                 in long clientXArg,
                                                 in long clientYArg,
                                                 in unsigned short buttonArg,
                                                 in EventTarget relatedTargetArg,
                                                 in DOMString modifiersList,
                                                 in long wheelDeltaX,
                                                 in long wheelDeltaY,
                                                 in long wheelDeltaZ);
};

wheelDeltaX is a number indicating the horizontal distance (positive means
rotated to the right, negative means rotated to the left).

wheelDeltaY is a number indicating the vertical distance (positive means rotated
away from user or to the right, negative means rotated towards user or to the
left).

wheelDeltaZ, included for future extensibility, is a number indicating the
distance along a third unspecified axis (positive means rotated away from the
user, means rotated towards user). At the present time, implementors may use
whatever modality is best suited to the task, such as the the use of the wheel
in conjunction with a control key.

The default value of wheelDeltaX, wheelDeltaY and wheelDeltaZ is 0.
UIEvent.detail must always be 0.

This event includes both scroll deltas. The default action of this event is to
dispatch a <code>mousewheel</code> event if the y delta is non-zero. Besides
that, its default action is to do whatever platform conventions suggest for the
wheeling behavior.

The <code>mousewheel</code> event implements the following interface:

interface MouseWheelEvent : MouseEvent {
   readonly attribute long            wheelDelta;
   void               initMouseWheelEventNS(in DOMString namespaceURI,
                                            in DOMString typeArg,
                                            in boolean canBubbleArg,
                                            in boolean cancelableArg,
                                            in views::AbstractView viewArg,
                                            in long detailArg,
                                            in long screenXArg,
                                            in long screenYArg,
                                            in long clientXArg,
                                            in long clientYArg,
                                            in unsigned short buttonArg,
                                            in EventTarget relatedTargetArg,
                                            in DOMString modifiersList,
                                            in long wheelDelta);
};

If the <code>mousewheel</code> event is cancelled, only the default action for
vertical wheeling is cancelled. 

    wheelDelta
        A number indicating the distance, measured as the number of "clicks" the wheel has been rotated. A positive value indicates that the wheel has been rotated away from the user (or in a right-hand manner on horizontally aligned devices) and a negative value indicates that the wheel has been rotated towards the user (or in a left-hand manner on horizontally aligned devices). The default value of the wheelDelta attribute is 0.

        A "click" is defined to be a unit of rotation. On some devices this is a finite physical step. On devices with smooth rotation, a "click" becomes the smallest reported amount of rotation.


XXX: What about UIEvent.detail?

For both <code>mouseomniwheel</code> and <code>mousewheel</code>,
<code>MouseEvent.relatedNode</code> must indicate the element over which the
pointer is located, or <code>null</code> if there is no such element (in the
case where the device does not have a pointer, but does have a wheel).