The site specifies a base font size of 12px. The better practice is to not specify a base font size at all, just taking it from the user's web browser instead. Then, the web designer should specify every other font size and box dimension as a scaled version of the base font size, using units like em/rem/%, not px.

Related reading: https://joshcollinsworth.com/blog/never-use-px-for-font-size