<script type="text/javascript">
         window.onload = function(){
             var oTab = document.getElementById("tabs");
             var oUl = oTab.getElementsByTagName("ul")[0];
             var oLis = oUl.getElementsByTagName("li");
             var oDivs= oTab.getElementsByTagName("div");

             for(var i= 0,len = oLis.length;i<len;i++){
                 oLis[i].index = i;
                 oLis[i].onclick = function() {
                     for(var n= 0;n<len;n++){
                         oLis[n].className = "";
                         oDivs[n].className = "hide";
                     }
                     this.className = "on";
                     oDivs[this.index].className = "";
                 }
             };
         }
    </script>

var oUl = oTab.getElementsByTagName("ul")[0];在这里为什么有[0]呢，不就有一个ul吗