Draw a curve with css
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>@Navaneeth and @Antfish, no need to transform you can do like this also because in above solution only top border is visible so for inside curve you can use bottom border.
.box {
width: 500px;
height: 100px;
border: solid 5px #000;
border-color: transparent transparent #000 transparent;
border-radius: 0 0 240px 50%/60px;
}<div class="box"></div>