网页中实现倒计时程序

<script language="JavaScript">
		var t = 3;//设定跳转的时间
		setInterval("refer()", 1000); //启动1秒定时
		function refer() {
			if (t == 0) {
				location = "javascript:window.history.back()"; //#设定跳转的链接地址
			}
			document.getElementById(‘show‘).innerHTML = "<div style=‘margin-left:25%;‘><h1><span style=‘color:red;‘>操作失败<br/></span></h1>"
					+ t
					+ "秒后跳转回去<a href=‘javascript:window.history.back()‘>重新操作</a>！</div>"; // 显示倒计时
			t--; // 计数器递减
		}
	</script>
	<span id="show"> </span>