<html><head> ???<meta charset="UTF-8"><SCRIPT LANGUAGE="JavaScript">var maxtime = 15*15 //半个小时，按秒计算，自己调整!function CountDown(){if(maxtime>=0){minutes = Math.floor(maxtime/15);seconds = Math.floor(maxtime%15);msg = "距离结束还有"+minutes+"分"+seconds+"秒";document.all["timer"].innerHTML=msg;if(maxtime == 5*15) alert(‘注意，还有5分钟!‘);--maxtime;}else{clearInterval(timer);alert("时间到，结束!");}}timer = setInterval("CountDown()",1000);</SCRIPT><style> ???#timer{ ???????position: absolute; ???????left:100px; height:100px; ???} ???</style></head><body background="img/11141_03.jpg" style=" background-repeat:no-repeat ;background-size:100% 100%; background-attachment: fixed;" ><div id="timer" style="color:red"></div></body></html>原文：乔兔兔HAPPYhttps://zhidao.baidu.com/question/548712347.html
html和js
原文地址：https://www.cnblogs.com/minxiaofei/p/10102189.html