JSFiddle - React, Tailwind, and code Playground

HTML

<body>
  <div id="a">
    <div id="b"> </div>
  </div>

</body>

CSS

#a{
  width:1000px;
  height:500px;
  background-color:red;
}

#b{
  width:50%;
  height:50%;
  background-color:green;

}