I know that to display embedded fonts in (i)Books on Apple you need to add
com.apple.ibooks.display-options.xml to epub2 or insert
<meta property="ibooks:specified-fonts">true</meta>
in the OPF for epub3.
However, I found this is also necessary even if you are just using system fonts, like
.sans {font-family:sans-serif;}
I had some text like this, and and that worked fine:
<p><b class="sans">TEXT IN SANS</b></p>
But if I wrapped several paragraphs in divs:
<div class="sans">
<p> Blah....</p><p> Blah....</p>
</div>
The font was ignored in iBooks.
I also had a few paragraphs set in monospace:
p.digits{text-align:center;font-weight:bold;margin:1em 1em;font-family:monospace;font-size:1.1em;}
<p class="digits">12345</p>
These displayed correctly most of the time, but in one chapter they were in the default serif, though still centred and bold.
In all of the above, the text showed the intended font in Sigil, and also on Kindle after conversion to AZW.
After adding the XML file and loading to iBooks, font display is now correct in all cases.
(In Sigil, use this plugin:
https://www.mobileread.com/forums/sh...d.php?t=272241 )
So, the moral is, just always include Apple's font enabling code in every ePub if it ever might be loaded onto an Apple device.