Given that the reasons why bug 13390 isn't going to be fixed means the widgets can't be read only I guess it would make sense to have :read-write applying to them.
However, some of the input types for which @readonly doesn't apply shouldn't be considered :read-write like hidden, button, submit and reset. The ones that should be :read-write are: range, color, checkbox, radio and file.

I think the specs should add as a second item after: "The :read-write pseudo-class must match any element falling into one of the following categories:" (4.14.2, ":read-write"), the following case:
"input elements to which the readonly attribute does not apply and with a value mode different from value."
If the value mode seems wrong, we could list the types for which it applies or the types for which it doesn't.