It appears that in HTML5, the contents of the role attribute is specified as a WIA-ARIA attribute. See, specifically, the last paragraph of section 3.2.3 "Global attributes", where it states:
To enable assistive technology products to expose a more fine-grained interface than is otherwise possible with HTML elements and attributes, a set of annotations for assistive technology products can be specified (the ARIA role and aria-* attributes).
The role attribute is, then, further detailed in section 3.2.7 "WAI-ARIA", and provides an apparently normative table of semantics and values for the attribute on an element-by-element basis.
Note: The links to specific sections of the HTML5 specification above point to the "latest published version" of the HTML5 Technical Report at http://www.w3.org/TR/html5/. A more current "editor's draft" version might be available. The TR will link to it on the title page of the document. As the specification is updated or transitions to a new phase of adoption, these links might break and will need to be updated.
