编写雪花js特效无显示

<script>
    //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()
    }
</script>

.snow{
        background: white;
        position: absolute;
        width: 20px;
        height: 20px;
        border-radius: 50%;
     }

mWebView.getSettings().setJavaScriptEnabled(true); //启用js
mWebView.getSettings().setBuiltInZoomControls(true);//启用缩放