<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
h1{
font-size: 50px;
text-align: center;
}
</style>
</head>
<body>
<h1>倒计时:<span>50</span></h1>
<script type="text/javascript" src = "
http://libs.baidu.com/jquery/1.10.0/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var e = $("h1 span").first();
var i = 49;
var interval = setInterval(function(){
e.html(i);
i--;
if(i <= 0){
clearInterval(interval);
}
},1000);
})
</script>
</body>
</html>