Return a reference to the positionable element whose stacking z-order is above or below the current element. These properties operate in the context of all positionable elements in a document. If the current element is the highest element, the above property returns null. To restrict the examination of next higher or lower elements within a single layer context, see siblingAbove and siblingBelow. To adjust the stacking order with respect to specific objects, see the moveAbove( ) and moveBelow( ) methods.