JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="child">
  
  </div>
</div>

CSS

* {
  box-sizing: border-box;
}

.parent {
  width: 150px;
  height: 150px;
  background: red;
  display: flex;
  align-items: center;
  justify-content: center;
}

.child {
  width: 50px;
  height: 50px;
  background: black;
}

.parent > .child {
  opacity: .5;
}