Something

<script type="text/javascript">
function hover() {
var navItems = document.getElementById("menu").getElementsByTagName("li");
for (var i=0; i<navItems.length; i++) {
if(navItems[i].className == "submenu") {
navItems[i].onmouseover=function() { this.className += " hover"; }
navItems[i].onmouseout=function() { this.className = "submenu"; }
}
}
}
window.onload = hover;
</script>