JSFiddle - React, Tailwind, and code Playground

HTML

<main>

  <div class="d1"></div>
  <div class="d2"></div>

</main>

CSS

* {
  box-sizing: border-box
}

html,
body {
  height: 100%;
  padding: 0;
  margin: 0;
}

main {
  display: flex;
  flex-direction: column;
  height: 100%;
  border: 1px solid green;
}

.d1 {
  flex: 1;
  background: gray;
}

.d2 {
  height: 200px;
  background: red;
}