There are many tests that test the existence of interfaces where duck-typing should be employed.
ex. The geolocation tests test if the code can create a Coordinates object.
The spec says that the Coordinates object has certain properties, but it does NOT specify that window.Coordinates is a function, or that typeof on a returned Coordinates object should be of type Coordinates.
We should be testing solely on the existence of the properties defined in the spec.
Realistically, there is no reason for a developer to create a Coordinates object as none of the APIs expect this object.