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Per ARIA Roles model spec (http://www.w3.org/TR/wai-aria/roles#treeitem), the structure of a tree is described as a parent/container with role of tree, items with role of treeitem, and groups of collapsable/expandable treeitems can be enclosed in elements with role group. This would seem to suggest construction in DOM as nested UL>LI>UL>LI..., with corresponding roles tree>treeitem>group>treeitem..., but the list of allowed roles for UL disallows group, so currenly disallows this. The list of allowed ARIA roles for UL/OL/LI are specified in: * http://dvcs.w3.org/hg/aria-unofficial/raw-file/tip/index.html * http://www.w3.org/html/wg/drafts/html/master/dom.html#sec-implicit-aria-semantics An LI can be a treeitem; a treeitem can have a tree or group as its parent; the parent of a LI can be either a UL or OL; this would suggest that UL or OL should in turn be tree or group, but the above specs disallow UL or OL from having role of group, and only allow: directory, list, listbox, menu, menubar, tablist, toolbar, tree. The NU validator flags <ul role="group"> as being in error, likely as a result of this. This issue arose as a result of a StackOverflow qu about the validator behavior - http://stackoverflow.com/questions/14616615/html5-validation-on-aria-trees/14622336 (If it is actually the case that the spec is correct here, and that UL/OL should not have a role of group, then this raises the question of what elements should be used to represent the items in a nested portion of a tree; could no longer use LIs, since it must have UL or OL as a parent.)
I've added "group" as allowed role for UL elements. The HTML5 spec currently still does not list group as an allowed role for OL, so it should probably be added there first before changing it here.
(In reply to Hans Hillen from comment #1) > I've added "group" as allowed role for UL elements. The HTML5 spec currently > still does not list group as an allowed role for OL, so it should probably > be added there first before changing it here. group role now allowed on ol in html5