font-size 62.5
Setting font-size in a root element (HTML) to 62.5% gives 10px base value ( on the assumption that a dafault font in browser is set to 16px).
See the Pen font-size: 62.5 by majadc (@majadc) on CodePen.
Bad approach is setting in a root element font-size to 10px what that is not letting user to freely scale a font size in their browser.
See the Pen OJVYYKY by majadc (@majadc) on CodePen.