Change Proposal for not including longdesc=""
The longdesc="" attribute does not improve accessibility in practice and should not be included in the language.
Several studies (listed in the references) have been performed. They have shown that:
- The longdesc="" attribute is extremely rarely used (on the order of 0.1% in one study). 
- When used, longdesc="" is extremely rarely used correctly (over 99% were incorrect in a study that only caught the most obvious errors ; the correct values were below the threshold of statistical significance on another study that examined each longdesc="" by hand ).
- Most users (more than 90%) don't want the interaction model that longdesc="" implies. 
- Users that try to use longdesc="" find it doesn't work ("Who uses this kind of thing? In my experience [...] it just didn't work. There was no description.") .
Furthermore, there already exist a number of alternative mechanisms for providing information to users without using longdesc="", such as simply including the information inline, providing explicit links to long descriptions, and using ARIA attributes such as aria-describedby="".
Including the longdesc="" attribute in the language therefore seems like a poor design decision.
No change to the spec.
- Stops authors from spending time trying to use a feature that they don't understand and that users don't want.
- Encourages authors to include suitable information in an alternative form that is more likely to be accurate.
- Results in better overall accessibility on the long term.
Conformance Classes Changes
No change to spec.
This would not affect existing ATs and user agents, as they can continue to support longdesc="" if compatibility with some set of documents where it is used correctly is desired. In practice, removing support is likely to either not be noticed (some users don't know the feature exists) or actually improve matters (given how poorly the feature is used in practice on the Web).
ARIA provides a number of alternative mechanisms that are currently not poisoned by existing content and that fit better into the kind of interaction model desired by users (according to the survey cited above). For example, aria-describedby="" allows an image to be related to in-page descriptive content.
Links included inline.