Fixes #19460 (closed).
This removes our fixed 16px font-size variable and it's usage throughout the project, allowing web visitors to utilize their customized browser default font-size. Visually, there is no difference between this change and what's been in v4 since we started it. The browser default is usually 16px, so you should be good to go there. In other places, and in case of customize root font-size, your content should scale nicely given the use of rems for type scale.
/cc @keithjgrant @patrickhlauke