JavaScript实现5秒倒计时自动跳转页面

<!DOCTYPE html>
<html>
 <head>
 <title>浏览器对象</title> 
 <meta http-equiv="Content-Type" content="text/html; charset=gb123"/> 
 </head>
 <body>
 <H4>操作成功</H4>
 <p>
 <b id="second">5</b>秒后回到主页&nbsp;<a href="javascript:goBack();">返回</a> 
 </p>
 
<script type="text/javascript"> 
 
 var sec = document.getElementById("second");
 var i = 5;
 var timer = setInterval(function(){
 i--;
 sec.innerHTML = i;
 if(i==1){
  window.location.href = "//www.jb51.net";
 }
 },1000);
 
 function goBack(){ 
 window.history.go(-1);
 } 
 </script> 
 </body>
</html>