verbose image descriptor requirements

aloha!

so that we are all working towards the same end (no matter the specific 
means) i have put up a list of requirements and some thoughts about how 
they can be achieved using the toolkit currently at the disposal of 
the HTML WG

http://www.w3.org/WAI/PF/HTML/wiki/Verbose_desc_reqs

i have attempted to keep the requirements few, clear and simple, but 
if you have one i haven't listed or have a terser, clearer means of 
expressing any of the requirements, please do so...  as of 2010-08-13 
0037 UTC the page contained the following content:

Verbose (As Long As Necessary) Descriptor Requirements

1. A way to reference a specific set of structured content, internal 
  (enhanced describedby model) or external (HTML4 longdesc model) 
  to the document containing the described image;

2. A way to inform all users that the descriptive content is   
  present/available;

3. A device independent way for all users to access the descriptive 
  content;

4. An explicit provision that accessing descriptive content, whether 
  internal or external to the document containing the image, does 
  NOT take the user away from the user's position in the document 
  containing the image where the verbose descriptor was invoked;

5. A way to provide user control over exposition of the descriptor 
  so that rendering of the image and its description is not an 
  either/or proposition;

Satisfying These Requirements for HTML5

Question: are these options or steps?

* retain support for longdesc; allow for exposition of longdesc inline 
 as well as for simultaneous exposition of both the image and its 
 description (useful for those with very limited viewports or users 
 with cognative issues, who may need a description's guide to assist 
 in the user's understanding of the image being described); 
     * advantage: 2 major browsers already support longdesc natively 
       and are expected to continue to do so as part of their support 
       for HTML4x;

* add support for aria-describedby and deprecate longdesc in HTML5; 
     * drawback: aria-describedby is currently limited to text that 
       appears in the same document as the image being described; 
     * drawback: The content associated using aria-describedby as 
       currently implemented, is limited to unstructured text;

* add support for external references and structured text to the 
 aria-describedby attribute in HTML5 and deprecate longdesc;

Related Resources 
[1] HTML 5 Issue: Image Equivalent Content (HTML WG Wiki page) 
   http://www.w3.org/html/wg/wiki/LongdescRetention

[2] HTML WG Issue-130 (longdesc) 
   http://www.w3.org/html/wg/tracker/issues/30

[3] Chairs' decision on HTML WG Issue-130 (longdesc) 
http://lists.w3.org/Archives/Public/public-html/2010Aug/att-0112/issue-30-decision.html

[3.1] cover letter for Chairs' decision on HTML WG Issue-130 (longdesc) 
     http://lists.w3.org/Archives/Public/public-html/2010Aug/0112.html