<html>
<body>
<span id="secSpan"> 3 </span>
</body>
</html>
<script type = "text/javascript" >
// 总秒数
var time = 3;
// 倒计时
function timing() {
time--;
if ( time == 0 ) {
location.href = "________";
}
var secSpan = document.getElementById( "secSpan" ); // 获取指定标签，予以显示
secSpan.innerHTML = time;
}
// 计时开始
window.onload = function() {
setInterval( "timing()", 1000 );
}
</script>
原文：https://www.cnblogs.com/bear-19970220/p/10993486.html