Screen Coordinates of a element, via Javascript

window.screenX/Y are not supported on IE. But for other browsers, a close approximation of position is:

var top = $("#myelement").offset().top + window.screenY;
var left = $("#myelement").offset().left + window.screenX;

Exact position depends on what toolbars are visible. You can use the outer/innerWidth and outer/innerHeight properties of the window object to approximate a little closer.

IE doesn't provide much in the way of window properties, but oddly enough, a click event object will provide you with the screen location of the click. So I suppose you could have a calibration page which asks the user to "click the red dot" and handle the event with

function calibrate(event){
    var top = event.screenY;
    var left = event.screenX;
}

Or possibly use the mouseenter/leave events using the coordinates of that event to calibrate. Though you'd have trouble determining if the mouse entered from the left, right, top, or bottom.

Of course, as soon as they move the screen you'll need to recalibrate.

For lots more on browser property compatibilities see PPK's Object Model tables.

I disagree with jvenema's answer.

I have found a solution that is working for me on google chrome anyway, but I hope that this will works on many other browsers too (maybe not on some old internet explorers).

Inside the <script> tags declare new two global variables called: pageX and pageY with the var keyword.

These global variables should always store the coordinates of the top left corner of the rendered page relative to the top left corner of the screen monitor, but in order to achieve this goal, the window.onmousemove should refers to a function that sets the pageX to the difference of event.screenX and event.clientX, and the pageY to the difference of event.screenY and event.clientY.

Then all what you need to do is to get the coordinates of an element (relative to the top left corner of the rendered page) and add pageX and pageY. The results of these expressions will be the coordinates of this element relative to the top left corner of the screen monitor!

Code example (only javascript):

<script>
    var pageX;
    var pageY;
    window.onmousemove = function (e) {
       pageX = e.screenX - e.clientX;
       pageY = e.screenY - e.clientY;
    }
    function getScreenCoordinatesOf(obj) {
       var p = {};
       p.x = pageX + obj.clientLeft;
       p.y = pageY + obj.clientTop;
       return p;
    }
</script>

Hope that helps!