<div class="mr-border-radius"></div>

div {
  width: 300px; 
  height: 150px;
  left: 100px;
  top: 100px;
  background: #FF4E50;
  position: absolute;
  border-radius: 50% / 100% 100% 0 0;
  -webkit-animation: silly 1s infinite alternate;
  -moz-animation: silly 1s infinite alternate;
  -o-animation: silly 1s infinite alternate;
  animation: silly 1s infinite alternate;
}

@-webkit-keyframes silly {
  to {
    border-radius: 50%; 
    top: 50px;
  }
 }
 @-moz-keyframes silly {
  to {
    border-radius: 50%; 
    top: 50px;
  }
}
 @-o-keyframes silly {
  to {
    border-radius: 50%; 
    top: 50px;
  }
}
 @keyframes silly {
  to {
    border-radius: 50%; 
    top: 50px;
  }
}