代码：
<!DOCTYPE html>
<html>
<head>
<title>日落西山</title>
<meta charset="utf-8">
<style type="text/css">
.BK{height: 800px;border: 1px solid black; position: relative;overflow: hidden;animation: bj 10s linear infinite}
.sun{height: 300px; 300px;border-radius: 100%;background-color: red; position: absolute; top: 800px;animation: light 10s linear infinite;}
@keyframes light{
0%{box-shadow:0px 0px 100px white;transform: translate(0px,0px)}
20%{box-shadow:0px 0px 200px white;transform: translate(200px,-200px)}
40%{box-shadow:0px 0px 200px yellow;transform: translate(400px,-400px)}
50%{box-shadow:0px 0px 200px yellow;transform: translate(500px,-500px)}
60%{box-shadow:0px 0px 100px yellow;transform: translate(600px,-400px)}
80%{box-shadow:0px 0px 100px yellow;transform: translate(800px,-200px)}
100%{box-shadow: 0px 0px 200px white;transform: translate(1000px,0px)}
}
@keyframes bj{
0%{background-color: black;}
20%{background-color: #C4C2C7;}
50%{background-color: white;}
80%{background-color: #C4C2C7;}
100%{background-color: black;}
}
</style>
</head>
<body>
<div class="BK">
<div class="sun"></div>
</div>
</body>
</html>