JSFiddle - React, Tailwind, and code Playground

HTML

<body style="margin:0">
    <div style="height:100%;display:flex;flex-direction:column">
        <div style="background:#d0d0ff">
            This is a header
        </div>
        <div style="background:#d0ffd0;flex-grow:1; display: flex;">
            <div id='newdiv' style="display:flex;">
                <div style="background:#ffd0d0; display: flex;"> Col 1 </div>
                <div> Col 2 </div>
            </div>
        </div>
    </div>
</body>

CSS

html, body { height: 100%; }