Chris says that the loading of sub-resources is a different matter. Well, the most common silent failure I have is not due to status codes per se, but because of my DNS-based adblocker, many sub-resources fail to load (GET or POST) with a network error (TCP RST). Unless a blocked domain corresponds to the front page I visit in a URL, the unavailable resource represents a silent failure, and I have foolishly contacted Support desks multiple times because my own use of the adblocker caused a silent failure that was difficult to diagnose.
In fact, I now have no way of knowing what web apps are like without an adblocker. Because all my devices use the same configuration, and it's non-trivial to disable it, I simply don't know what I am missing. Indeed, in the case of one web app which I've used as a PWA, it was simply missing features that I knew should have been there, and whitelisting enough domain names caused those features to re-appear.
I will just leave you with the most adorable 404 page anyone has ever seen: https://diopitt.org/pray/amen