Curved background en afronding

.curvedbg 
  {
    width: 100%;
    height: 250px;
    background-color: rgba(0, 0, 0, 0.3);
    /* You can use gradient background color such as
    background: linear-gradient(rgba(0, 0, 0, 0.2) 0%, rgba(0, 0, 0, 0.1) 100%); */
    border-bottom-left-radius: 50% 40%;
    border-bottom-right-radius: 50% 40%;
  }

echo "<div class='curvedbg'></div>";