Thread:Citrusellaeditswikis/@comment-28672580-20160722074611/@comment-24155515-20160722151125

Alt text is a description for a screen reader to use to describe images to its user. It may also load when an image fails to load. In old versions of Internet Explorer, it used to also be treated like an HTML "title" attribute (which isn't really its intended purpose). The title attribute provides tooltip text like what you described.

It's totally possible to add the popover text you describe to an image, too, but I was inspired to start adding alt text to images after I came across an inline math image that I didn't think a screen reader would read in a way that was understandable. Then I went all over (perhaps a bit overexcitedly, even?) and did some alt text on non-math images.