> Are you suggesting that there is a default default font

Yes, the `font-size` value when you freshly install the browser: 16px. This changeable value had a lot of historical importance, as it was the only way users could scale sites up.

Good Samaritan CSS authors had to write not only all font-sizes in percentages or em's to respect that value (rem came later) but also think about things like min/max-widths, padding/margin, breakpoints, etc. Folks would later use pixel-to-em converter functions in early tools like Bourbon and Compass. You can still find old polemics on authoring explicit pixel values from folks like Jakob Nielsen[0].

These days, Cmd-+ in browsers zooms instead of scales font-size, and things just work out nicely: padding and margins magically grow, breakpoints trigger as expected, etc. As a result, direct font-size adjustments have gone from living in the taskbar in IE4 to being buried in Google Chrome in Preferences > Appearance > Font Size.

In my opinion, it's for the best. Users aren't left out in the cold by nonconforming CSS, and CSS devs don't have to do battle with scalable values.

[0] https://www.nngroup.com/articles/let-users-control-font-size...

I think the suggestion is that instead of using your computer with a font size that is comfortable to read, you should leave all the rest of the fonts on your system tiny, but zoom in your browser so that web pages are readable.

> Are you suggesting that there is a default default font that is expected to be used across all browsers?

Yes – 16px times new roman.

Almost everything overrides the default font size because almost every browser has a default of 16px, while almost every desktop OS uses a UI font of <13px.