How to make font-size relative to parent div?

The way I solved this problem was to use javascript to measure the container and then set the font size in px on that container. Once that baseline is set for the container then the relative font sizing of all the content will scale correctly using em or %.

I’m using React:

<div style={{ fontSize: width / 12 }} >
  ...
</div>

CSS:

div {
  font-size: 2em;
}