Xkcd uses alt text or titles, can't remember which, to pop up an accessibility tooltip. Clever and fun but it's quite dated and doesn't work on most (all?) mobile browsers.

Using mobile safari, at least, you can tap and hold to see the alt text.

Ah yeah but that's not very good typical UX, is it? To have to rely on the save menu to see alt text.

Definitely lacking in discoverability.