Transfers 1,4-N-acetylgalactosamine (GalNAc) from UDP-GalNAc to the non-reducing end of glucuronic acid (GlcUA). Required for addition of the first GalNAc to the core tetrasaccharide linker and for elongation of chondroitin chains. Important role in chondroitin chain biosynthesis in cartilage formation and subsequent endochondral ossification (PubMed:11788602, PubMed:12163485, PubMed:12446672, PubMed:17145758, PubMed:31705726). Moreover, is involved in the metabolism of aggrecan (By similarity). {ECO:0000250|UniProtKB:Q8BJQ9, ECO:0000269|PubMed:11788602, ECO:0000269|PubMed:12163485, ECO:0000269|PubMed:12446672, ECO:0000269|PubMed:17145758, ECO:0000269|PubMed:21160489, ECO:0000269|PubMed:27599773, ECO:0000269|PubMed:31705726}. This is the function of CSGALNACT1 (chondroitin sulfate N-acetylgalactosaminyltransferase 1, ENSG00000147408).