OK this warrants some explanation.
Here’s the thing.
Things can canonically appear in the comic without TECHNICALLY being canon.
As a matter of fact they do all the time! This isn’t bad, and it’s actually something I really enjoy about the comic. It really goes hand in hand with Andrew’s little “interpret the characters how you see fit” thing.
But lets just remember here. There are at least a dozen different visual artists who have contributed to Homestuck, and they all have very different interpretations of what the characters look like.
Let’s just for a second take a look at how many different ways Karkat has appeared.
Only two of those are drawn by Andrew Hussie. These are all things that have canonically appeared in Homestuck, but they aren’t necessarily how they “canonically look” because how things “canonically look” is up for interpretation however you see fit essentially. So getting bent out of shape or exclaiming “CANON!!!” because another artist, cosplayer, etc. had work appear in the comic doesn’t fully make sense because it’s really just adding on to the long long long list of ways characters have been canonically interpreted. Even Andrew Hussie himself has drawn the characters a TON of different ways!!!
Basically what I’m saying is, unless something is a story point, the canon is basically made of the wonderful jelly goo that we call “how to visually interpret homestuck characters”