JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

body {
  height: 500px;
  width: 500px;
  overflow: hidden;
}

#a {
  height: 100px;
  width: 100px;
  background-color:red;
}

#b {
  height: 100%;
  width: 100%;
  background-color:blue;
}