JSFiddle - React, Tailwind, and code Playground

HTML

<header></header>
<div id="content"></div>

CSS

html, body{
    height: 100%;
    margin: 0;
}
header{
    background: #fc0;
    height: 100px;
}
#content{
    background: #f00;
}

JavaScript

var header = document.getElementsByTagName('header')[0];
var content = document.getElementById('content');
window.onresize = refreshContent;

function refreshContent() {
    content.style.height = document.body.offsetHeight - header.offsetHeight + 'px';
}
refreshContent();