Why isn’t it ‘‘des’’ there? Which difference would that make?
Here “de” is used as in the form “de (undefined article) + epithet adjective + noun” which is the correct way of using de/des.
Note: The sentence is kind of weird, feels like the punctuation is wrong and should be more either :
Using de (undefined): Il y a de belles villes au Japon. Kyoto et Nara par exemple.
Try to understand it as: There are beautiful cities in Japan but I don’t know which ones. There are, that’s it. Oh Kyoto & Nara actually, just remembered.
Using des (defined): Il y a des belles villes au Japon comme Kyoto et Nara par exemple.
Try to understand it as: There are beautiful cities in Japan that I know/have in mind. And I’ll give you the examples of Kyoto & Nara.
thanks a lot for the clarification!