#quiz_start_panel {
background-image: url(/dom/assets/exercise_start.png);
width: 40vw;
height: 21vw;
background-size: 40vw 21vw;
position: absolute;
top: 50%;
left: 50%;
margin-left: -20vw;
margin-top: -11vw;
}
图片居中
原文：https://www.cnblogs.com/jason-beijing/p/10328408.html
