"undefined" value allowed to use?

[JAWS-test]

Some attributes have the default value "undefined". May this value be used? I think not, but I have not found any statements on this in the ARIA specification and "undefined" is used as a valid value in the ACT rules.

Example: aria-checked=undefined

• At roles where aria-checked is allowed (e.g. checkbox) it makes no sense because one of the three values (true, false, mixed) must be used.
• All roles where aria-checked is not permitted (e.g. button) have aria-checked=undefined implicitly, but this must not be set explicitly because the attribute is not permitted

See: #2078 (comment) and https://www.w3.org/WAI/standards-guidelines/act/rules/6a7281/proposed/#passed-example-3

[JAWS-test]

I also think the addition of "default" to "undefined" is wrong:

• "default" is only correct for elements where the attribute is not allowed
• "default" is sometimes a different value for elements that allow the attribute, e.g. "false" or "true" for aria-selected