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HTML radio buttons (<input type="radio">) can be in an indeterminate state, when none of them is selected. In HTML-AAM we could potentially map that to the "mixed" state. Either way, it should be clearly documented what to do with radio buttons in that state.
HTML AAM is no longer an ARIA Deliverable. If you wish to pursue this issue please file at https://github.com/w3c/html-aam/issues