JSFiddle - React, Tailwind, and code Playground

HTML

<header>
  HEADER CONTENT<br>
  HEADER CONTENT - LINE #2
</header>

<main>
  MAIN CONTENT
</main>

CSS

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

header {
  position: sticky;
  top:0;
  background-color: red;
  color: #fff;
}

main {
  background-color: green;
  color: #fff;
  min-height: 150vh;
}