JSFiddle - React, Tailwind, and code Playground

HTML

<button onclick="foo.textContent+=1">pew</button>

<div id="foo">zz</div>

CSS

#foo {
  font-size: 50px;
}

JavaScript

var target = document.querySelector('#foo')

var observer = new MutationObserver(
  function(mutations) {
    mutations.forEach(function(mutation) {
      var size = 50 - target.textContent.length
      target.style.fontSize = size + 'px'
    });    
  }
);

observer.observe(target, { childList: true });