下雪css:

<style>
        *,body,html{
            margin: 0;
            padding: 0;
            overflow: hidden;
            height: 100%;
            /*background: black;*/
        }
        .snow{
            background: white;
            position: absolute;
            width: 20px;
            height: 20px;
            border-radius: 50%;
        }
    </style>

下雪js：

//获取屏幕宽高
        var windowWidth = window.screen.width;
        var windowHeight = window.screen.height;
 
        //创建雪花
        function createSnow(){
            var left = 0;
            var top = 0;
 
            //定义一个初始化随机数,使雪花在屏幕中
            var left_random = Math.random() * windowWidth;
            var top_random = Math.random()* windowHeight;
            var div = document.createElement('div');
            div.className = 'snow';
            div.style.transform = 'scale('+(Math.random())+')'
 
            document.body.appendChild(div);
 
            //雪花飘落
            setInterval(function () {
 
                div.style.left = left_random + left +'px';
                div.style.top = top_random + top +'px'
                left += 0.2;
                top += 0.2;
 
                //如果雪花跑到屏幕外面了,让雪花重新返回屏幕顶部
                if(left_random + left >= windowWidth){
                    left_random = Math.random();
                    left = 0;
                }
 
                if(top_random + top >= windowHeight){
                    top_random = Math.random();
                    top = 0;
                }
            },10)
 
        }
        for(var i = 0 ; i < 200 ; i++){
            createSnow()
        }