I’m kinda tired of seeing blog posts advocate
html { font-size: 62.5%; }
In case you’re not aware, this makes the default font size for your entire website 10px (assuming the browser’s default font size is 16px, which is the standard but is also user-configurable). Sure, it makes the math easier when you’re working with relative font sizes from that point on (either ems or the more future-friendly rems), but it kinda ignores the point of setting a default font size in the first place.