[bes]

<style>
  #div1 {background: green}
  #div2 {background: yellow}
</style>

<div id="div1">div1
  <div id="div2">div2
   </div>
</div>

<script>
window.onload = function () {
  var div1 = document.getElementById('div1');

  div1.onmouseout = function (e) {
    e = e || window.event;
    var rt = event.relatedTarget || event.toElement;

    while (rt && rt !== this) rt = rt.parentNode;

    if (rt == this) return; else alert('hy');
  }

}
</script>