JSFiddle - React, Tailwind, and code Playground

HTML

<div id="test"></div>

CSS

body {
  margin: 0;
}

#test {
  width: 50vw;
  height: 50vh;

  background: tomato;
  display: flex;
  align-items: center;
  justify-content: center;
}

JavaScript

const el = document.getElementById('test')

el.innerHTML = 'hello'

const ro = new ResizeObserver(entries => {
	console.log('trigger resize')
	for (let entry of entries) {
    el.innerHTML = `${entry.target.offsetWidth} x ${entry.target.offsetHeight}`
  }
})

ro.observe(el)