Safari has been known to be dumb about image formats to pick from when deciding support/fallback, so to stop it from insisting on using formats it doesn't understand (esp. pre-Ventura/iOS17) the type attribute helps it to pick a suitable format.

While there are a lot of working images, I also see a lot of broken ones randomly checking in Safari 15 — and it seems to correlate with the markup used in the occurrences:

This one works:

This one renders broken ("missing") images:

(confirmed by inspecting the images and checking for presence/absence of the type attributes.)

It would seem that monkey-patching a (faux-)MIME type invented from the alt_format passed to high_res_image() helper could be added to the element as a type:

as a quick fix, if accepting that any alt_format "quux" would emit type="image/quux" just blindly…