[gampleman]

I think this lack of ability to distinguish screen reader users also comes with some serious drawbacks. I've been in situations (I do a lot of heavy data viz work) where providing alternative content for impaired users as well as graphical content for sighted users is a performance no go. If I could reliably detect which kind of content is preferred, then it would not be a problem. But if >90% of your audience is to suffer choppy frame rates (and 100% of your bosses) just to generate invisible content, the end result is that there is no alternative content (or crappy one: "A chart showing the relationship between instance count and cost" <- this leaves out all the interesting stuff).