startList = function() {
      if (document.all&&document.getElementById) {
            navRoot = document.getElementById("nav");
            for (i=0; i<navRoot.childNodes.length; i++) {
                  node = navRoot.childNodes[i];
                  if (node.nodeName=="LI") {
                        node.onmouseover=function() {
                              this.className+=" over";
                        }
                        node.onmouseout=function() {
                              this.className=this.className.replace(" over", "");
                        }
                  }
            }
      }
}
window.onload=startList;

 <ul id="nav">
                     <li class=""><a href="#">Home</a></li>
                     <li class=""><a href="#">About</a>
                         <ul>
                             <li><a href="#">History</a></li>
                             <li><a href="#">Team</a></li>
                             <li><a href="#">Offices</a></li>
                         </ul>
  </ul>