function dropdown_initialize() {
  var elements = Common.getElementsByClass('submenu');
  for(var i=0; i<elements.length; i++) {
    var container = elements[i].parentNode;
    container.onmouseover = function() {
      this.className = 'hover';
    };
    container.onmouseout = function() {
      if(container.className == 'hover') {
        this.className = '';
      }
    };
  }
}

<ul>
<li>abc
<ul class='submenu'>
<li>a</li>
<li>b</li>
<li>c</li>
</ul>
</ul>