Forcing users to listen to long descriptions is an extremely negative and harmful user-experience as John Foliot has explained,
The ability to (mentally and literally) pause, step outside of the page flow to get a description of a complex image (because you cannot see it) and then return to the content flow AT EXACTLY THE SAME PLACE YOU LEFT OFF is a design feature, not a flaw. The key point about @longdesc (for screen readers) is that they are given a *choice* as to whether or not they want to hear what some might consider extraneous data or not - it is the difference between glancing at a sophisticated pie chart (for example) versus studying it. You, as a sighted user, have that choice (to glance or study), yet insisting that the full-on textual description be inserted into the content flow because the user is blind is tantamount to me holding your head in a fixed position and insisting that you explain aloud to me that pie chart before I allow you to continue reading the rest of the page.
@longdesc is about user-choice!
aria-describedby the description is forced
upon screen reader users whether they want it or not. It is read
aloud without any user intervention, forcing the screen reader user
to listen to it each and every time they encounter the image
whether they want to or not. The user is not able to control how
they interact with the long description.
As the Association of American Publishers states,
aria-describedbycannot be silent by default in screen readers when used on images, compromising its use to illustrate that the content of an image is already available on the page.
In addition AAP goes on to explain what a confusing user experience this can create,
Developers may not realize the distracting and frustratingly circular user experience that this would cause and might use
aria-describedbyto point to, for example, a paragraph just above the image. Users would then likely follow the
aria-describedbyannouncement, expecting to find additional content, but they would arrive, instead, at a paragraph that they have likely just read.
Such behavior makes
aria-describedby a non-starter.
None of this is a problem with
longdesc as it supplies
descriptions on-demand and not by force. Non-sighted users (using a screen reader that supports
longdesc) are presented with a) advice/notification that a longer description is available, and b) the opportunity/choice to either pursue that longer description, or skip past it and continue with the normal page content. This choice is a critical user-requirement.