If the fonts aren't installed, you may see boxes. That's system-specific and user-specific and app-specific. I agree with you that it's pretty weak circa 2017. You may be right that boxes have been completely eliminated at this point, but I am not sure sure.
At some point, you just lose. If I send you Arabic text and you don't have an Arabic font, unless you plan to do something heroic like send it to a faraway rendering service that returns a png image, what are you going to do?
Of course. But the main point is that most modern systems are able to display a good deal of the commonly used scripts without extra downloads. My iPad, configured for dead-standard English, can still display Wiki pages in Russian and Greek and Hebrew and Arabic and Chinese and Japanese and Korean and.... If I want a nicer Chinese font, I can deal with that myself, but the basic support is there and I don't see boxes. I even get Mongolian and Thai. b