layerX / layerY / offsetX / offsetY

Two Functions for Retreiving the Position of the Mouse Relative to the Current Element
MSDN Definition

event.offsetX: Sets or retrieves the x-coordinate of the mouse pointer's position relative to the object firing the event.

MDC Definition

event.layerX: Returns the horizontal coordinate of the event relative to the current layer.

// Get the X position of the mouse relative to the element target
// used in event object 'e'
function getElementX( e ) {
    // Find the appropriate element offset
    return ( e && e.layerX ) || window.event.offsetX;
}
// Get the Y position of the mouse relative to the element target
// used in event object 'e'
function getElementY( e ) {
    // Find the appropriate element offset
    return ( e && e.layerY ) || window.event.offsetY;
}