JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrapper">
      <div id="header">Header</div>
      <div id="faux">Faux</div>
</div>

CSS

* { margin: 0; padding: 0; }

      html, body { height: 100%; background: #ccc; }
      body { padding: 20px; box-sizing: border-box; }

      #wrapper { width: 100%; background: #fff; min-height: 100%; }
      #header  { height: 80px; }
      #faux    { background: #ff0; }

JavaScript

$("#faux").height($("#wrapper").height() - 80);