I have created my own ePub reader using a UIWebView on iOS and I've made many attempts to get the fonts to render correctly, but they won't. I've placed a test ePub's unzipped contents on a publicly accessible web server and just loaded it in various apps--two on macOS (iBooks and Safari), and two on iOS (Mobile Safari and a custom app using UIWebView). Here is the resulting output:
The top two render acceptably--best in iBooks and second in Safari on macOS. So what gives on iOS? It looks like it has substituted the font even though the font is available just the same as it is in Safari on macOS. The kerning (horizontal distance between characters--or is it tracking) looks all wrong--ostensibly because it's substituting some other font (not sure about that).
I've tried adding the Slate Pro font explicitly in my UIWebView based custom app in the proscribed way with a reference in the plist and that doesn't (seem to) load the font either.
Any ideas what might be missing to allow the app the find the fonts?