Round the top-right corner:
border-top-right-radius: 50px;

Stretch it out one way...
border-top-right-radius: 150px 50px;

Or go crazy with every corner to make a deformed box:
border-top-left-radius: 150px 30px; border-top-right-radius: 100px 50px; border-bottom-left-radius: 40px 90px; border-bottom-right-radius: 20px 75px;