Catalyzes the first step in the biosynthesis of chondroitin sulfate, heparan sulfate and dermatan sulfate proteoglycans, such as DCN. Transfers D-xylose from UDP-D-xylose to specific serine residues of the core protein. {ECO:0000269|PubMed:17189265, ECO:0000269|PubMed:26027496}. This is the function of XYLT2 (xylosyltransferase 2, ENSG00000015532).