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The details element has a default implied role of group and a restriction that "Role must be a role that supports aria-expanded" http://dev.w3.org/html5/spec/wai-aria.html#wai-aria The 'group' role cannot have aria-expanded attribute applied. Roles that can are button, document, link, separator and window. None of which appear appropriate for the details element. What is specified for details would be more appropriate for the summary element. As in implementations (webkit/chrome) this is the interactive element and is most likely to be implemented as a button in some API's that don't have a disclousre triangle role. Recommend removing summary form the first table and changing the row in the second table that lists details under "Language feature" to the summary element, leaving all the rest of the text that pertianed to details the same. Recommend adding 'details' to the first table with a default implied role of "group"
my bad, misreading of the ARIA spec, so closing bug. still think the details/summary element mappings need tweaking, will file a new bug.