When a user’s own letter avatar doesn’t load, discourse doesn’t show any placeholder for it. That means the user’s “profile/messages/preferences” menu at the top-right completely disappears, preventing them access to many features. Typing a P will bring it up, but first-time users won’t even know anything is missing, and looking-up those keyboard shortcuts is too much to expect of users in general. This problem also shows up elsewhere, like making it impossible to show which users have “Liked” a post, as the elements collapse, preventing mouse-over.
I suspect that this could be most quickly and easily fixed by merely filling-out the alt= field for the avatar images, instead of leaving them all null. Including a small screenshot made in the sandbox, for those who haven’t seen this effect first-hand:
I’d like to sidestep all the reasons why avatars don’t always load (as a dozen other topics have), and focus on ensuring the UI is more robust to that eventuality. Whether it’s network issues, CloudFlare issues, local caching issues, or even the user choosing to disable image loading in the browser. I don’t believe image load failures should break the functionality of a website.