Credit to Thierry Koblentz: http://cssmojo.com/aspect-ratio-using-custom-properties-and-calc/
Who credits Sérgio Gomes: https://www.npmjs.com/package/css-aspect-ratio
I'm always at least a generation behind with my code and take a while to adapt. I'm glad I looked at this pen. This gave me a good laugh because it's so intuitive. I should start using CSS variables.
Need to know how to enable it? Go here.