CSS設置元素沿着Y軸旋轉

<div class="open"><em><img src="open.png" /></em></div>

.open em img { width:100px; height:100px; border-radius:50%; animation:rotate .8s infinite; -webkit-animation:rotate .8s infinite;}
@-webkit-keyframes rotate{
    from{ -webkit-transform:rotateY(360deg);}
    to{ -webkit-transform:rotateY(0deg);}     
}
@keyframes rotate{
    from{ -webkit-transform:rotateY(360deg);}
    to{ -webkit-transform:rotateY(0deg);}     
}