I'd like to learn more about how alt text is used and how to properly construct alt text in a way that is helpful and usable. I've been looking for advice but regrettably most of the sites I'm finding are not helpful. Many sites are marketing scum talking about how to use alt text for SEO. Others are government or corporate compliance sites that are clearly describing how to meet minimal legal compliance obligations.

Could anyone point me at some information from people who use alt text explaining what is helpful for them? I'd like to know what kind of formatting works well for text-to-speech services and other assistance systems, what level of detail and description is appropriate, and especially how people actually use alt text when they're browsing web sites; What kind of information would a user like to have to get the most out of the website they're viewing?

Any help would be appreciated. Cheers!