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In Chapter 8.3 of XSL 2.0 requirements there is list with several print specific properties. I would add there also screening info, which could be applied to every block element. Specification could be similar to PDF reference at http://www.adobe.com/devnet/acrobat/pdfs/pdf_reference_1-7.pdf (Chapter 6.4) and also Named Halftone dictionaries could be supported. Later mentioned are described in section 6.4.4 (p.496): "Halftone dictionaries have an optional entry, HalftoneName, that identifies the halftone by name." Such information could be transferred into PDF file and if such file would be processed for offset printing on modern CTP devices, every object could be rasterized with different settings which could even allow to combine halftone and stochastic screening on single plate and significantly improve final quality (for specific jobs).