Chrome Gmail unread messages not appearing in bold

We had a user whose unread emails were appearing as bold in Safari (with an ugly font) and not bold at all in Chrome (font looked okay), and we did some digging and found the solution was to enable the Arial Bold font in Font Book on the user's Mac, and it was all good.

Our best guess is that Safari couldn't find the font it wanted so substituted an ugly font but kept it bold, and Chrome couldn't find the font it wanted so substituted a decent-looking font but not bold.