*{ margin:0; padding:0;}
.aa{ width:100px; height:100px; background:#000;animation:hehe 3s ease infinite;-webkit-animation:hehe 3s ease infinite;}
@keyframes hehe
{
0%   { margin-left:0}
50% { margin-left:200px}
100% { margin-left:0}
}
@-webkit-keyframes hehe
{
0%   { margin-left:0}
50% { margin-left:200px}
100% { margin-left:0}
}