The Unsupported features in CSS 2.1 appendix of the User Guide says that “Prince does not support the quotes property or the open-quote and close-quote values.”

Is this because these values or the quotes property are difficult to support? They are very useful when nested quotes are required, or when support for many languages is necessary.

It would be desirable, I think, to have Prince support these values and the quotes property, and to have it use the HTML5 specification’s default quotes values to have nested quote support for all the languages listed there.

While the full behaviour of the quotes property probably can't be exactly replicated by user CSS, I think it can be done close enough for practical purposes. Unless your documents contains dozens of languages and deeply nested quotations, in which case I would have to recommend processing it with JavaScript to assign quote styles.