addition of HTML role requirement mappings

Hi all,

hixie has added some role requirements and restrictions in the whatwg spec
that will  soon come up for cherry picking [1] to the HTML 5.1 spec.
previously we determined that some of the following were not appropriate,
so am asking for ARIA folk to review:

note: implementers MUST map to these roles and authors are restricted to
applying the roles listed for each element (authors can also apply
role=presentation).

strong semantics:

table element     grid role
tbody element     rowgroup role
td element     gridcell role
tfoot element     rowgroup role
thead element     rowgroup role
tr element     row role


implicit semantics:

th element     gridcell role     Role must be either columnheader,
rowheader, or gridcell



[1]
https://github.com/w3c/html/commit/5b5ad72b8e7bb36ab863effc942829eef82fe3c4#commitcomment-5449710















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Regards

SteveF
HTML 5.1 <http://www.w3.org/html/wg/drafts/html/master/>