<html>
<head>
<meta charset="utf-8">
<title>index</title>
<style>
*{margin: 0;padding: 0;}
#myCircle{margin: 200px auto;width: 300px;height: 300px;border-radius: 50%;text-align: center;}
#timeLeft{margin:0 auto;line-height: 300px;font-size: 25px;font-family: black;}
</style>
</head>
<body>
<div id="myCircle">
<div id="timeLeft">
</div>
</div>
</body>
<script>
var int = window.setInterval(doSomeThing,1000);
function doSomeThing(){
// 随机变色
var red, green, blue, transParent, myColor;
red=Math.floor(Math.random()*256);
green=Math.floor(Math.random()*256);
blue=Math.floor(Math.random()*256);
transParent = 1-Math.random();
myColor = "rgba("+red+","+green+","+blue+","+transParent+")";
document.getElementById('myCircle').style.background = myColor;
// 倒计时
var timer = new Date();
myMin = 60 - timer.getMinutes();
mySec = 60 - timer.getSeconds();
document.getElementById('timeLeft').innerHTML="倒计时 |    "+myMin+"  分"+mySec+"  秒";
}
</script>
</html>-------------------------------------------
运行效果图
-------------------------------------------
总结：
1、Math.random()方法的返回值含0不含1，如果要含1不含0，则用1减去方法的返回值；
2、要取到【M，N】（均为正整数），要做如下计算：Math.floor(Math.random()*(N-M+1)+M)
3、window.setInterval(代码，间隔)，如果代码是一个方法或函数，无需使用双引号