div:hover
{
-webkit-animation: myfirst 5s; /* Chrome, Safari, Opera */
    animation: myfirst 5s;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes myfirst {
    from {margin-top:10px;}
    to {margin-top:30px;}
}

/* Standard syntax */
@keyframes myfirst {
    from {margin-top:10px;}
    to {margin-top:30px;}
}