JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header">Create a div that stretches across the window, with a 50px gap between both sides of the div and the edges of the window:</div>
<div class="main"></div>

CSS

html,body {
  height: 100%;
}
.header {
  background: yellow;
  height: 50px;
}
.main {
  background: green;
  height: calc(100% - 150px);
}