Body font best practice for flexibility

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I'm looking for some help on editing books so I can display the text in either the embedded, publisher font, or else in a user chosen font.

It often annoys me when epub books lock you into a particular font face and font size. I'm capable of editing epubs with Sigil and editing body tags or div and paragraph tags.

But I can't seem to figure out the rules for what "publisher font" even means on Kindle and Kobo. I find that if fonts are embedded and used for the main text (ie: not just titles or limited, special formatting) then I usually can't change it on the reader.

Is there a tutorial on this somewhere? I thought I saw something relevant in the wiki years ago, but can't dig it up again.

What I want:

- to have a base epub as my archival copy. I usually read on a Kobo Aura One. When I am on my Kindle Paperwhite, I use Calibre to convert on sending to the device
- to have the option to use whatever font face and font size the publisher chose for the main text
- to also have the option to choose my own font and size on the fly