JSFiddle - React, Tailwind, and code Playground

HTML

<div class="div1">
  <div class="div2">
  </div>
</div>

CSS

.div1 {
  background-color: red;
  width: 300px;
  height: 400px;
}

.div2 {
  background-color: green;
  width: 300px;
  height: 400px;
  border-top-left-radius: 400px 200px;
  border: 3px solid #000;
}