Ask HN: Minimal CSS for text pages readable on all current devices

Once upon the time the pages with no CSS were possible and were readable. Example: https://www.bell-labs.com/usr/dmr/www/ Today, this page has too small font on my iPhone to be readable. So how to keep the CSS minimal and allow pages made like that to be readable on all current devices? Sounds simple, still I don't know the answer. But somebody on HN surely knows?