@keyframes ex4_anim {
0% { border-radius: 200px 30px 30px 30px; }
25% { border-radius: 30px 30px 30px 200px; }
50% { border-radius: 30px 30px 200px 30px; }
75% { border-radius: 30px 200px 30px 30px; }
100% { border-radius: 200px 30px 30px 30px; }
}
#cercle {
border:2px solid #999;
background-color: #ccc;
width: 200px;
height: 200px;
margin:auto;
box-shadow: 0 0 70px black inset;
animation : ex4_anim 2s infinite alternate;
}
<div id="cercle"></div>