Self-tutoring about the Chrome browser: the tutor measures its default font size in pixels.
I opened the Chrome settings and found its font set to “medium”.
I then made a quick page with two divs: one set at 20em, then the other at the pixel width so it matches the first. The matching pixel width turns out to be 320px. Importantly, I didn’t define font size anywhere on the page. Doing so can prejudice the results.
An em, typically is “one character.” Therefore, if 20em=320px, then 1em=16px. On this browser, anyway, the default font size is 16px.
Source:Oracle Tutoring by Jack and Diane, Campbell River, BC.