JSFiddle - React, Tailwind, and code Playground

HTML

<p>window.getComputedStyle</p>

CSS

p {
  font-size: 12px;
}

JavaScript

var root = document.querySelector("p"),
  style = window.getComputedStyle(root, null).getPropertyValue('font-size'),
  fontSize = parseFloat(style);

root.style.fontSize = (1 * fontSize) + 'px';