A lot of things in the series, some of which were stated since the beginning, have been confirmed as canon in the show.
In "Chapter 9: I Ain't Troubled Yet!", Luz asks Principal Bump if his head demon, Frewin, is supposed to serve as glasses, which he states is unimportant at the time. Then "Hunting Palismen" aired and revealed that Frewin is also Bump's Palisman and really does help him see due to his damaged sight.
"Them's the Breaks, Kid" also depicts Gilbert Park being in the construction track when he was a student like in the fic (although here Gilbert's the African-American based dad while in canon, he's the Korean based one).
In "Chapter 52: No Dancing Around", Amity decides to become a wild witch due to learning the truth about the Emperor and becoming nameless, thus shaking off all of her mother's influence. 2 weeks later, "Reaching Out" aired and she's come to the same decision in the show.
Hooty demonstrates early on that he has a Lockdown mode for security reasons, a fact that gets unintentionally confirmed in "Hollow Mind".
Another example in "Hollow Mind" is the reveal that Hunter was a coven scout before he became the Golden Guard, a fact that was shown in "Chapter 55: Deserve Nothing".
"Chapter 12: Heart to Heart" shows that Willow's dads taught her a technique for panic attacks, which she canonically demonstrates in "Labyrinth Runners".
"Chapter 56: Going Through Changes" has Kikimora getting demoted for her recent actions in tegards to her acting behind the Emperor's back, something that comes to pass in "Clouds on the Horizon".
Also in "Clouds on the Horizon", Luz promises to take Amity on a mundane, Slice of Life date with no drama or monsters to deal with. This was the very plot of "Chapter 54: Socket to Me".
"Chapter 82: Shaping Up or Nothing" shows that Lilith dyed her hair so it's the original red color she had before joining the Emperor's Coven. Almost 2 weeks later (disregarding the leak), the trailer for "For the Future" shows that her hair naturally returned to red after months evading the Collector.
"Chapter 71: The Hunting Witch" marks the debut of a bird palisman named Waffles, better known as Dell's yellow cardinal. When the finale special "Watching and Dreaming" aired, a character named 'Waffles' actually appears (although it's Hunter's blue jay palisman bearing the name.)
"Chapter 74: Hexing Out for a Hero" has Nina insult Amity by saying she has a big forehead. Months later, MoringMark published this comic.
In "Chapter 87: A Tale of Two Palismen", the Collector questions if Philip Clawthorne is a dragon upon seeing him in his cambion form. Chapter 108 would later reveal that Belos would end up being the ancestor of one of the last dragons on the Boiling Isles, AKA Cross. In a manner of speaking, this also counts as Harsher in Hindsight.
"Chapter 88: The Return of the Witches" introduces Blazer, a dalmatian Hellhound. Come the Helluva Boss episode, "Queen Bee", we see some dalmatian Hellhounds.
The same episode shows Loona to be Ambiguously Bi. The first chapter of the spinoff series, which was published 5 days earlier, shows her to be this due to her attraction to Kanya.
Chapter 77 ends with Nicole burning her home down to take revenge on Athena for, in her own mind, wronging her. Over a year later, Moringmark posted the comic "Jay" which has Boscha tell her daughter of when her pink-haired mother committed arson to her siblings' businesses because they actually wronged her.
"Road to Redwood Highway" ends with Vaggie arriving in Heaven and meeting Adam and Lute. Six months later, the Hazbin Hotel episode "Welcome to Heaven" reveals that she's a Fallen Angel.
.