window.onload = function () {
      // 编写JS代码
            var list = document.getElementsByTagName("li");
            for(var i=0; i<list.length; i++){
                list[i].onmouseover = function(){
                    this.className = "lihover";
                }
                list[i].onmouseout = function(){
                    this.className = "";
                }
            }
        }