[Quoth]

Originally Posted by repilo

Another example would be a book about ancient Rome, with chapter titles and drop caps set with some ancient font (which I don't want to change) and the general text with a serif that I would be able to change when I feel like it.
That would be nice, Kobo, if you're listening.

Mediaeval Germany might be a better example

This was the Roman Alphabet 2000 years ago
A B C D E F Z G H I K L M N O P Q R S T V X . They had no upper and lower case. That's close to the letter style used. C was pronounced K, like in Irish today, hence Caesar -> Kaiser. J was very much later simply I (or a Y sound) at the start of a word. No J.
Lower case was invented in Ireland 600 years later, and was used in Ireland till about 120 years ago. It's similar to current lower case. It became Carolingian minuscule

German Blackletter was used later, from about 11thC to 1941 when Hitler discontinued it (no-one is completely bad, just 99.99% bad).
People might use a Fraktur family font for Blackletter headings of Gothic stuff. The Old English font is related and nothing to do with Old English. It's slightly more readable than German Blackletter.
Blackletter, especially as body font, is very inaccessible.

The User Font selection should have an Advanced setting where you pick fonts for sans, serif, monospace, cursive and decorative and have the option to leave headings unchanged.

The "apply this font to everything" sledgehammer is lazy accessibility. Serif as a body and Sans as headings is a common choice. People used to web sites might want sans for body. People most easily read what they are used to.

Generic Font Families

In CSS there are five generic font families:

Serif fonts have a small stroke at the edges of each letter. They create a sense of formality and elegance.
Sans-serif fonts have clean lines (no small strokes attached). They create a modern and minimalistic look.
Monospace fonts - here all the letters have the same fixed width. They create a mechanical look.
Cursive fonts imitate human handwriting.
Fantasy fonts are decorative/playful fonts.

Good font declarations in CSS should also declare one of the five generic fonts as well as the desired font. Font families often have Sans and Serif versions. Cursive is not Italic or Oblique.

.p1 {
  font-family: "Times New Roman", Times, serif;
}

.p2 {
  font-family: Arial, Helvetica, sans-serif;
}

.p3 {
  font-family: "Lucida Console", "Courier New", monospace;
}

You can just have the preferred and the generic.
https://www.w3schools.com/Css/css_font.asp

It's managers and programmers that don't seem to understand accessibility.