document.getElementsByClassName = function(cl) {
    var retnode = [];
    var myclass = new RegExp('\\b'+cl+'\\b');
    var elem = this.getElementsByTagName('*');
    for (var i = 0; i < elem.length; i++) {
        var classes = elem[i].className;
        if (myclass.test(classes)) retnode.push(elem[i]);
    }
    return retnode;
}



var nav_link = document.getElementsByClassName("nav_li");
for(var i=0; i<nav_link.length; i++){
	nav_link[i].onmouseover=function(){
		this.className="onover";
		}
	nav_link[i].onmouseout=function(){	
		this.className="nav_li";
		}
		
	}