Per Jonas:

To construct a key range a set of constructors are available. In
languages with interface objects [WEBIDL], these constructors are
available on the IDBKeyRange interface object. In other languages
these constructors are available through language specific means, for
example as static functions"

But I see that I forgot to say that the IDBKeyRangeConstructors
interface implements these constructors.

However WebIDL nowadays have support for "static" so there is no
longer a need for a separate interface. We should just do

interface IDBKeyRange {
   readonly attribute any     lower;
   readonly attribute any     upper;
   readonly attribute boolean lowerOpen;
   readonly attribute boolean upperOpen;

   static IDBKeyRange only (in any value);
   static IDBKeyRange lowerBound (in any bound, in optional boolean open);
   static IDBKeyRange upperBound (in any bound, in optional boolean open);
   static IDBKeyRange bound (in any lower, in any upper,
                                in optional boolean lowerOpen,
                                in optional boolean upperOpen);
};