For now only values that have been set using the DOM property can be read using the DOM property.
Not all ARIA properties will be implemented, notably ARIA properties related to ID references, like aria-labelledby, aria-controls, etc..., since the HTML specification is still defining how properties using ID references will be implemented in the DOM.
The value returned from a ARIA DOM property should be a string, but if it was set with a boolean value it may be returned as boolean value (e.g. aria-checked).