Ok, I can't imagine blind people able to use a mouse very well. So I also imagine they have a brail keyboard. But does that mean they are set up with speakers and a keyboard and they learn how to navigate a computer really slowly, and that modern webpages are very.... Noisy?
Markdown features are extremely fragmented. Hover text might be a non-standard feature that not all markdown renderers can handle (or even a standard feature that's omitted in some renderers).
I am using Thunder, if I tap the image for both of yours (to make full screen) I can see the alt text on both. Not while I am in the post though, scrolling through replies.
I may have to tap on more images from here on. Thanks to you both for the info and examples!