document.getElementById('clickme').onclick = function clickEvent(e) {
      // e = Mouse click event.
      var rect = e.target.getBoundingClientRect();
      var x = e.clientX - rect.left; //x position within the element.
      var y = e.clientY - rect.top;  //y position within the element.
      console.log("Left? : " + x + " ; Top? : " + y + ".");
    }