If I'm right, Prince does not currently support the quotes property and q {quotes: ....} is ignored. I guess I can simulate it with q::before and q::after (and for the different levels of quotes something like q q::before etc), but it would be handy if Prince supported directly the property. Thank you!

Right, Prince does not support the quotes property at the moment. You can use ::before and ::after as you suggest, or even JavaScript.