An em is historically based off the width of the letter M in a typeface. https://iamvdo.me/en/blog/css-font-metrics-line-height-and-vertical-align https://iamvd

An em is historically based off the width of the letter M in a typeface. https://iamvdo.me/en/blog/css-font-metrics-line-height-and-vertical-align https://iamvdo.me/en/blog/css-font-metrics-line-height-and-v... is a fun deep dive into font sizing, which is pretty hairy.