<div class="wrap">
  <div class="main"></div>
</div>
.wrap {
  display: flex;
  justify-content: center;
  margin: 40px auto 0;
}

.main {
  background: #0aaaa0;
  border-top-left-radius: 100px 150px;
  border-top-right-radius: 100px 150px;
  width: 200px;
  height: 150px;
}

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.