Out of curiosity I tried out the new malala theme available on pressbooks.com. I looked at a book on my iphone with iOS13. I noticed that the justified text had many large “holes”. I first thought that this is an unavoidable consequence of the text-align justified. But then I noticed that the “holes” arise because no hyphenation is happening on the iphone despite the hyphens: auto. I then added -webkit-hyphens: auto to the custom css and the justified text looked nice on the iphone too. Apparently iOS (also the latest version 13) only knows -webkit-hyphens. So maybe it makes sense to add -webkit-hyphens auto to the right place in the theme css (or in buckram?). Maybe include the -ms prefixes too?
Thanks for the suggestion @t.dumm. I’ll add this as a ticket to the private Malala repository.