JSFiddle - React, Tailwind, and code Playground

HTML

<p>Some paragraph text</p>
    <h1>some heading 1 text</h1>
    <h2>some heading 2 text</h2>

    <a href="#" id="size-12">12</a>
    <a href="#" id="size-14">14</a>
    <a href="#" id="size-16">16</a>

CSS

body {
  font-family: Helvetica, Arial, sans-serif;
  font-size: 12px;
}

h1 {
  font-size: 1.5em;
}
h2 {
  font-size: 1.2em;
}

JavaScript

function makeSizer(size) {
    document.body.style.fontSize = size + 'px'
}


document.getElementById('size-12').onclick = makeSizer.bind(0, 12);
document.getElementById('size-14').onclick = makeSizer.bind(0, 14);
document.getElementById('size-16').onclick = makeSizer.bind(0, 16);