JSFiddle - React, Tailwind, and code Playground

HTML

<div id="header"><span>WIDTH</span></div>

CSS

#header {
    position: absolute;
    left: 0px;
    right: 0px;
    border: 1px solid black;
}

JavaScript

var epsilon = 5;
var div = document.getElementById('header');
var span = div.childNodes[0];

function size() {
    var dw = div.offsetWidth;
    var fs = 200;
    span.style.fontSize = fs + 'px';
    var cw = span.offsetWidth;
    for (var i = 0; i < 10 && Math.abs(cw - dw) > epsilon; ++i) {
        fs *= dw / cw;
        span.style.fontSize = fs + 'px';
        cw = span.offsetWidth;
    }
}

window.onresize = size;
size();