css怎么设置元素一直旋转

<div class="demo">

<img class="an img" src="1.jpg" width="400" height="400"/>

.demo{text-align: center;

    margin-top: 100px;}

@-webkit-keyframes rotation{

    from {-webkit-transform: rotate(0deg);}

    to {-webkit-transform: rotate(360deg);}

}

.an{

    -webkit-transform: rotate(360deg);

    animation: rotation 3s linear infinite;

    -moz-animation: rotation 3s linear infinite;

    -webkit-animation: rotation 3s linear infinite;

    -o-animation: rotation 3s linear infinite;

}

.img{border-radius: 200px;}