document.onmousemove = handleMouseMove;
function handleMouseMove(event) {
var dot, eventDoc, doc, body, pageX, pageY;
event = event || window.event; // IE-ism
// If pageX/Y aren't available and clientX/Y
// are, calculate pageX/Y - logic taken from jQuery
// Calculate pageX/Y if missing and clientX/Y available
if (event.pageX == null && event.clientX != null) {
eventDoc = (event.target && event.target.ownerDocument) || document;
doc = eventDoc.documentElement;
event.pageX = event.clientX +
(doc && doc.scrollLeft || body && body.scrollLeft || 0) -
(doc && doc.clientLeft || body && body.clientLeft || 0);
event.pageY = event.clientY +
(doc && doc.scrollTop || body && body.scrollTop || 0) -
(doc && doc.clientTop || body && body.clientTop || 0 );
// Add a dot to follow the cursor
dot = document.createElement('div');
dot.style.left = event.pageX + "px";
dot.style.top = event.pageY + "px";
document.body.appendChild(dot);