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.)