JSFiddle - React, Tailwind, and code Playground

HTML

<div class="circle">
<div class="square">

</div>
</div>

CSS

.circle {
  border-radius: 50%;
  width: 100px;
  height: 100px;
  background: green;
  position: relative;
  overflow: hidden;
}
.square {
  width: 50px;
  height: 50px;
  position: absolute;
  bottom: 0;
  right: 0; 
  background: black;
}
body {
  background: black;
}