Hi Kovid - ok, I guess I still don't see how that stylesheet thing might work, please bear with me as I get up to speed on this.
Take for instance the epub that is on this thread here at MobileRead by JSWolf:
https://www.mobileread.com/forums/sho...d.php?t=177182
It has embedded fonts, a single stylesheet, and @font-face declarations in that single stylesheet. It clearly was converted by calibre to epub at one point, and then the embedded fonts and declarations were done afterwards, quite possibly with Sigil by the looks of the metadata. So we are starting with a "perfect" epub - it has font-face declarations in a stylesheet, that is referenced from every html page.
So if I do an epub->epub conversion on that with calibre currently, the @font-face declarations now get moved into every single html file except the titlepage.xhtml (along with copying the page margins, which I know cause similar angst to some people).
In our "ideal" world, that wouldn't happen. Since you don't want an option to turn that off, you suggest instead a stylesheet referenced from every page. But there is already exactly just that? So what is the difference? Does it have to be *another* stylesheet? With special naming or something?