The first of the two lines below is unstyled. The second line is styled with "font-size:medium". Given that this is a "standards"-mode page, medium should be the default font size and the two lines look identical. In the default configuration that's true, however, if the size preferences for "Webpage normal text" is increased sufficiently (at least 20 in my experience, but possibly even bigger) a "medium" font-size becomes slightly larger than the unstyled font-size.
abcdefghijklmnopqrstuvwxyz
abcdefghijklmnopqrstuvwxyz