animation:myfirst 5s infinite;
animation-direction:alternate;
-webkit-animation:myfirst 5s infinite;
-webkit-animation-direction:alternate;
0% {background:red; left:0px; top:0px;}
25% {background:yellow; left:200px; top:0px;}
50% {background:blue; left:200px; top:200px;}
75% {background:green; left:0px; top:200px;}
100% {background:red; left:0px; top:0px;}
@-webkit-keyframes myfirst
0% {background:red; left:0px; top:0px;}
25% {background:yellow; left:200px; top:0px;}
50% {background:blue; left:200px; top:200px;}
75% {background:green; left:0px; top:200px;}
100% {background:red; left:0px; top:0px;}
<p><strong>注释:</strong>Internet Explorer 9 以及更早的版本不支持 animation-direction 属性。</p>