html{height:100%;background-image:-webkit-radial-gradient(ellipse farthest-corner at center top,#000d4d 0,#000105 100%);background-image:radial-gradient(ellipse farthest-corner at center top,#000d4d 0,#000105 100%);cursor:url('http://testtp5.xyz/wp-content/uploads/2018/05/bitbug_favicon.ico'),auto}
body{width:100%;height:100%;margin:0;overflow:hidden}
iframe{top:0;left:0;height:100%;width:100%;position:absolute}
*{margin:0px;padding:0px; font-family: "微软雅黑";}可以适当添加z-index
附上一个国外大佬用js写的樱花飘落iframe
<iframe src="http://www.jsdaima.com/Uploads/js/201802/1517720362/index.html" allowtransparency="true" frameborder="0" width="100%" height="100%"></iframe>