You could use an asymmetrical border to make curves with CSS.
border-radius: 50%/100px 100px 0 0;
VIEW DEMO
.box {
width: 500px;
height: 100px;
border: solid 5px #000;
border-color: #000 transparent transparent transparent;
border-radius: 50%/100px 100px 0 0;
}<div class="box"></div>