[html] Summary draft

http://dev.w3.org/html5/pf-summary/Overview.html#the-table-element

Here is an attempt at finding a middle ground on @summary, an attempt to continue the dialog about this accessibility attribute. It is not perfect, and would benefit from further discussion.  Most of the edits are in 4.9.2 The table element http://dev.w3.org/html5/pf-summary/Overview.html#the-table-element

The highlights of this draft are this

1) It leaves the summary attribute in, using the HTML 4.01 text but edited to fit the sentence format of the HTML 5 spec.

2) It adds summary to the content attributes and DOM interface.  I added it at the end of the DOM interface, not sure that's the right style.  I also added it to the table API as table.summary, table.createSummary, and table.deleteSummary, but I'm not sure that's really needed.  Perhaps normal attribute accessors are enough?

3) It adds some text and a code example donated by Matt May from his and Wendy Chisholm's book Universal Design for Web Applications.  I still need to add the working table example, but do have the code snippet in there.

4) It describes at some length when to use summary vs. the techniques in Ian's draft.  This basically boils down to using summary for things that are obvious if you see them, and confusing if you can't.  Ian's examples are retained with minor edits to the surrounding text.  The text is still a bit clunky, but I think it suffices to open the dialog.  I also notice that there is a similar example under the caption element.  It reads a bit strangely to have it in two places, and would be nice to consolidate them.  I also plan to submit Ian's examples to WCAG, but haven't yet done that.

5) It adds some discussion of Platform Accessibility APIS and programmatically determined under WCAG, and when ARIA might be needed to make either of these work as expected.  I also added aria-describedby to the example that uses a paragraph before the table.  See http://www.w3.org/TR/WCAG20/#new-terms for a definition and discussion of programmatically determined under WCAG.

6) It adds a section on Guidance for conformance checkers, which suggests warning when any of these techniques is used, describing the difference between summary and the others, and also for any table which uses none of the techniques.  This essentially means warning on every table, which I'll admit seems less than ideal.  This is modeled after the guidance for alt in the image attribute.    I think this allows removing summary from the obsolete section, but still gives the opportunity to provide guidance to authors about when to (and not to) use it.  http://dev.w3.org/html5/pf-summary/Overview.html#guidance-for-conformance-checkers-0

7) It removes summary from the obsolete section

I'll be sending this to the HTML list, but wanted to get a bit of a sanity check and some initial discussion here first.

Thoughts?