Someone

function FindElementIn (x, y, e)
{
    if (x < e.offsetLeft || x > e.offsetLeft + e.offsetWidth)
	return null;
    else if (y < e.offsetTop || y > e.offsetTop + e.offsetHeight)
	return null;
    else {
	/* it's inside this element, try to find a more deeply nested one */
	for (var el = e.firstChild; el != null; el = el.nextSibling) {
	    var rv = FindElementIn (x - e.offsetLeft, y - e.offsetTop, el);
	    if (rv != null)
		return rv;
	}

	return e;
    }
}