The best font size to set in css is no size.
Fonts, too. Everyone has their favorite fonts, but the font a web designer likes has no bearing on what's best or easiest to read for the user. Are they developing a website as an art piece, or to publish articles for real people (not bots) to read?
Ancillary styling for text that needs to stand out, or text that isn't part of the main flow, can be whatever. But I don't even agree with shrinking footnotes anymore. There's no lack of page real estate at the bottom; there's no reason to shrink "back matter" except to imply that you don't expect anyone to ever read any of it (footnotes, index, bibliography, etc. in long-form blogs or books).
When the web was created, HTML was meant to semantically describe text documents. But that isn’t an accurate description of the web today. Today, websites have complete control over their own visual appearance.
Users not knowing how to change their font or font size is not an excuse for website designers to choose fonts and font sizes whimsically, since they rarely base their decisions on good quality readability data. Browsers and other UAs need to step up and provide meaningful interfaces to choose those defaults at install time, and make it easy (and easily discoverable) to adjust.