I posted about this here in the Item Inspector thread, but it really impacts Jitai: it seems that you simply cannot access locally-installed (non-system-standard) fonts in Safari/WebKit anymore without defaults modifications (though if you followed a macOS upgrade path, you may have had that turned on for you). Just webfonts and system-default fonts. And to subvert “font fingerprenting attacks”, this issue is going to spread to other browsers soon, too.
That would make Jitai in its current incarnation much less useful: Windows, macOS and most Linux and mobile distributions come with a Mincho, a Gothic, and a Rounded font, but beyond that you’re out of luck.
It seems like the longer-term solution is to support webfonts. Since we can’t upload webfonts to WaniKani, they would have to be in a public repository like Google Fonts.