JSFiddle - React, Tailwind, and code Playground

HTML

<div class="t1">
  My divider
</div>
<div class="t2">
  Inside the div
</div>

CSS

body {
  margin: 0;
  /* This is used to reset any browser-default margins */
}

.t1 {
  background: #f00;
  color: #fff;
  height: 20px;
}

.t2 {
  background: #212121;
  height: calc(100vh - 20px);
}