How to add fallback font style?

[franklin-deasis-alctech]

If the span tag does not have a font class assigned to it. Some (older?) devices are unable to render some special characters. They show up as [X]. (ə and ː). When a font class has been applied in span, it can render the characters properly

<p><span>/divéləp/</span></p> -> divél[X]p
<p><span>/míːn/</span></p> -> mí[X]n

Is there a way for the reader to display these characters even if the device's fallback font style cannot render these characters?

I did look at using something like below. But this would change the font style of everything in the Epub file. So this is not ideal.

epubNavigator.submitPreferences(EpubPreferences(
    fontFamily = FontFamily.LITERATA
))

I may just be missing some setting or am misunderstanding the cause of the issue. Any help is greatly appreciated

[mickael-menu]

Could you open a bug report issue for this one, with a sample EPUB showing the problem? Thank you.