nsIDOMNSDocument Interface

 Returns the element from the caller's document at the given point,
 relative to the upper-left-most point in the (possibly scrolled)
 window or frame.

 If the element at the given point belongs to another document (such as
 an iframe's subdocument), the element in the calling document's DOM
 (e.g. the iframe) is returned. If the element at the given point is
 anonymous or XBL generated content, such as a textbox's scrollbars, then
 the first non-anonymous parent element (that is, the textbox) is returned.

 This method returns null if either coordinate is negative, or if the
 specified point lies outside the visible bounds of the document.

 Callers from XUL documents should wait until the onload event has fired
 before calling this method.

 <a href="http://dev.w3.org/cvsweb/~checkout~/csswg/cssom/Overview.html?content-type=text/html;%20charset=utf-8#documentlayout-elementfrompoint">preliminary spec</a>

nsIDOMElement elementFromPoint(in long x, in long y)