.ballBox{
	position: fixed;
	left: 0;
	top: 0;
	z-index: 9;
	/*  用颜色来演示用原理 */
	/*background-color: #4CD964*/
	height:30rpx;
	width:30rpx;
}
.ballOuter {
   background:red;
   height:100%;
   width:100%;
   border-radius: 50%;
   background-size: 100% 100%;
   background-position: center;
}