 solid black;
}
#box {
  background-color: green;
  position: relative;
  offset-path: xywh(10% 10% 90% 90%) padding-box;
  offset-distance: 40%;
  border-radius: 50% 50% 0 0;
  width: 100px;
  height: 100px;
}
</style>

<div id="outer">
  <div id="box"></div>
</div>
