window.onload = function() {
   items = document.getElementById('menu').getElementsByTagName('li');
   for (i = 0; i < items.length; i++) {
      items[i].onmouseover = function() {
         submenu = this.getElementsByTagName('ul');
         if (submenu.length)
            submenu[0].style.display = 'block';
      }
      items[i].onmouseout = function() {
         submenu = this.getElementsByTagName('ul');
         if (submenu.length)
            submenu[0].style.display = 'none';
      }
   }
}