function menuHover(){
var menuItems = document.getElementById("nav").getElementsByTagName("li");
for (var i = 0; i < menuItems.length; i++) {
menuItems[i].onmouseover = function() {
this.className = "mnhover";
}
menuItems[i].onmouseout = function() {
this.className = "";
}
}
}

window.onload=function(){menuHover()}