The image you posted is an HTML template. If they did it, don'T you think it is possible?
It is, just put a border on bottom left and bottom right. One value is 100% and the other is the height you want the curve to stop :
border-bottom-right-radius : 100% 100px;
border-bottom-left-radius : 100% 100px;
Fiddle : http://jsfiddle.net/yqf9a/1/