How to create a curve on the top of a background?

.box {
  margin-top:90px; /* make it at lealst the same as the height of the pseudo element */
  width:200px;
  height:100px;
  background:white;
  position:relative;
}

.box:before,
.box:after{
  content:"";
  position:absolute;
  bottom:100%;
  width:50%;
  left:0;
  height:80px; /* adjust this to control the height */
  background:
    radial-gradient(50% 100% at bottom left, #fff 98%,#0000) top,
    radial-gradient(50% 100% at top right  , #0000 98%,#fff) bottom;
  background-size:100% 50%;
  background-repeat:no-repeat;
}
.box:after {
  transform-origin:right;
  transform:scaleX(-1);
}
body {
  background:pink;
}

<div class="box">
</div>