[AlanHK]

I have epubs that I edit in Sigil, then create an AZW using the kindlegen plugin, then sideload to my PW3.

I've noticed that some books the Kindle would not allow me to change the font: if I pulled up the font menu, I could select whatever I wanted, but the display did not change.

I stripped down the epub to try to work out the reason.
Eventually I got a to a stylesheet with one style:

p {
     font-family: sans-serif;
}

The above showed Ember. If I made it "serif" it showed Bookerly, regardless of my selection. I can change the size as normal.

Same behaviour if I had

body {
     font-family: sans-serif;
}

I attach the test epub.

But many other epubs I have use "font-family:", and the display font can be changed regardless.

Anyone can cast a light on what is going on and what triggers this behaviour?

I normally do put "font-family: serif;" in the body CSS as I found that otherwise if I had some text in an embedded font (e.g a Fraktur for a few words), and set "Publisher Font" on to see it, all other text became sans otherwise (ugh).

I really would prefer it if you could choose the "font unspecified" display font while leaving the explicitly set ones to show. But I don't see any prospect of that.