The IDB spec could be simplified by adding:

typedef (DOMString or sequence<DOMString>) IDBKeyPath;

And then using it in a handful of places:

interface IDBObjectStore {
  readonly attribute IDBKeyPath? keyPath; // replacing 'any'
  IDBIndex createIndex (DOMString name, IDBKeyPath keyPath, optional IDBIndexParameters optionalParameters); // replacing explicit union
}

dictionary IDBObjectStoreParameters {
    IDBKeyPath? keyPath = null; // replacing explicit union
};

interface IDBIndex {
    readonly attribute IDBKeyPath keyPath; // replacing 'any'
}

This would not change the defined behavior at all.