Catalyzes the first step in the biosynthesis of chondroitin sulfate and dermatan sulfate proteoglycans, such as DCN. Transfers D- xylose from UDP-D-xylose to specific serine residues of the core protein (PubMed:15461586, PubMed:17189265, PubMed:23982343, PubMed:24581741). Required for normal embryonic and postnatal skeleton development, especially of the long bones (PubMed:23982343, PubMed:24581741). Required for normal maturation of chondrocytes during bone development, and normal onset of ossification (By similarity). {ECO:0000250|UniProtKB:Q811B1, ECO:0000269|PubMed:15461586, ECO:0000269|PubMed:17189265, ECO:0000269|PubMed:23982343, ECO:0000269|PubMed:24581741}. This is the function of ENSG00000103489 (XYLT1, xylosyltransferase 1).