It seems to me that there are two common scenarios for alt text.
1) The alt text is a perfect substitute for the image. For example:
Replace IBM's logo with the word IBM.
Alt text for some sort of fancy graphical title or headline.
A set of hyperlinks which duplicate the function of an image
2) The alt text describes the image. For example:
A photo of me.
An image map that is a _real_ map.
Charts or figures.
Wouldn't it be valuable to distinguish between these two cases?
This would allow a more advanced `delayed image loading' for people on
slow links or strange hardware. I am sure there are other uses as well.
Of course, most people who use fancy graphical titles probably think
delayed image loading is an abomination anyway.