Use :root for flexible type

flexible-type.css

/* This has been added to CSS Protips https://github.com/AllThingsSmitty/css-protip */

/* The type font size in a responsive layout should be able to adjust with each viewport.
You can calculate the font size based on the viewport height and width using :root */
:root {
  font-size: calc(1vw + 1vh + .5vmin);
}

/* Now you can utilize the root em unit based on the value calculated by :root */
body {
  font: 1em/1.6rem sans-serif;
}