@jcacabelos we could certainly consider a PR for something like this if it is something you want to work up. That said, fonts can accept a wide variety of CSS font size units:
em|ex|ch|ic|rem|vw|vh|vi|vb|vmin|vmax|cm|mm|q|in|pc|pt|px
Some of these are supposedly relative to other elements in the document (though I don’t have experience with them myself) I would suggest first investigating whether using different units might also work, in which case we could simply document that.