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.
Still determining whether string versus boolean values (or depends on how it was set) will be returned for some states with boolean values (e.g. aria-checked).