CSS debugging tool: a visual `font-size` helper

index.css

[data-font-size]:after {
  background: rgba(255,50,190,.9);
  color: #fff;
  content: attr(data-font-size);
}

index.js

[...document.querySelectorAll('*')].forEach(el => {
  const styles = window.getComputedStyle(el);
  el.setAttribute('data-font-size', styles.getPropertyValue('font-size'));
});