Marcial 🇨🇷🇻🇪 on Nostr: For someone who maybe understands Mastodon better than I do, is there a reason for ...

For someone who maybe understands Mastodon better than I do, is there a reason for the checkered outline on the GIF attachment, despite there being ALT text on it? I always thought the checkered outline was to mark an picture without ALT text from a different instance.