原理：
雪花背景平铺，背景图片位置向右下移动的动画
缺点：不会永远都向下移动，动画时间结束会闪回去首帧
实现：
<!DOCTYPE html>
<html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>简单下雪</title><style>* {margin: 0;padding: 0;}body {background: rgb(41, 43, 41);}.snow {width: 100vw;height: 100vh;background-image: url("imgs/2.png");margin: 0 auto;animation: snow 100s linear;}@keyframes snow {0% {background-position: 0 0;}100% {background-position: 5000px 5000px;}}</style>
</head><body><div class="snow"></div>
</body></html>雪花图片：
