Edit in JSFiddle

<div class="radius-demo">
    border-radius: 40px / 20px;
</div>

div  {
    width: 300px;
    height: 100px;
    border: 1px solid #ccc;
    line-height: 100px;
    text-align: center;
}
.radius-demo {
    border-radius: 40px/20px;
}