function initializeDropDownMenu(){
// get 'nav' element
var nav=document.getElementById('nav');
 if(!nav){return};
// get all menu items
var menuitems=nav.getElementsByTagName('li');
if(!menuitems){return};
for(var i=0;i<menuitems.length;i++){
// assign 'over' class attribute to each menu item on 'mouseover'
menuitems[i].onmouseover=function(){
this.className+=' over';
}
// remove 'over' class attribute to each menu item on 'mouseout'
menuitems[i].onmouseout=function(){
this.className=this.className.replace(' over','');
}
}
}
// initialize drop-down menu when web page is loaded
window.onload=function(){
if(document.all&&document.getElementById&&document.getElementsByTagName){
initializeDropDownMenu();
}
}