Without retreading too much of what people smarter than I have already said, my opinion is that canon as a concept is only really useful when given unambiguously by an authority (whether that's an author or a few centuries of theological inertia) or decided on a personal basis to inform your own understanding. It's safe to say that we probably aren't getting an official unambiguous delineation of what's canon and what's not; and unlike, say, the Bible and its associated writings, Homestuck & Friends are more interested in exploring, playing with, and challenging the idea of canonicity than presenting an indisputable narrative of What Is True. This makes them about as easy to sort into canon categories (canongories) as mice are to sort into egg cartons. This also gives us, as fans, enough uncertainty that we will likely never agree on a true canon...

... is what someone with less pizzazz would say. I put forward this interpretation that I'm sure everyone will agree with:

Canon:
Specifically the following phrase from Page 18 of the Epilogues (Candy):
"Jake stares transfixed into the full moon of the clown’s buttocks [....]"

Postcanon:
Everything occurring chronologically afterwards: the rest of Candy, HSBC, etc.

Precanon:
Everything occurring chronologically beforehands: the preceding parts of Candy, Homestuck, Hiveswap, etc.

Dubiously Canon:
Everything occurring at a time or in a reality that cannot be, with certainty, placed relative to the clown's buttocks: Meat, Friendsim, nearly every other work of fiction ever made, etc.