Canon is whatever is released officially. By the Homestuck Company. Non-canon is all the fan-works. Canon is determined by the trademark and copyright. Any further discussions of canon are, in truth, discussions about how to invalidate, or validate, art. "Is everything the creator says is canon? Are the epilogues canon?" are code for "WHAT CAN I SAY TO HURT PEOPLE THE MOST AND NOT FACE REPERCUSSIONS?"