Considering how all the answers to this question say that there is no such thing as Harry Potter canon, but the answers to this question point out that Cursed Child has been called "canon", I would like to trying putting some context to Rowling's tweet.
Things J.K. Rowling has called Canon
- Snape is not a Vampire
JKR: No, I see that, and yeah, I follow your line there. I can't — I mean, obviously, there are lines of speculation I don't want to shut down. Generally speaking, I shut down those lines of speculation that are plain unprofitable. Even with the shippers. God bless them, but they had a lot of fun with it. It's when people get really off the wall — it's when people devote hours of their time to proving that Snape is a vampire that I feel it's time to step in, because there's really nothing in the canon that supports that.
("The Leaky Cauldron and MuggleNet interview Joanne Kathleen Rowling: Part Two")
- Hermione has "brown eyes, frizzy hair and is very clever."
Canon: brown eyes, frizzy hair and very clever. White skin was never specified. Rowling loves black Hermione
- The story of the Cursed Child
The story of #CursedChild should be considered canon, though. @jackthorne, John Tiffany (the director) and I developed it together.
- Everything in the Fantastic Beasts movies
Q. There were obviously some major changes from the HP books to the films... Are we to take everything in these films as canonical?
JKR: Yes, because I'm writing them!
Things J.K. Rowling has not called canon
- Harry Potter and the Philosopher's Stone
- Harry Potter and the Chamber of Secrets
- Harry Potter and the Prisoner of Azkaban
- Harry Potter and the Goblet of Fire
- Harry Potter and the Order of the Phoenix
- Harry Potter and the Half-Blood Prince
- Harry Potter and the Deathly Hallows
- Quidditch Through the Ages
- Fantastic Beasts and Where to Find Them
- The Tales of Beedle the Bard
- The eight Harry Potter movies
- Any of the videogames or other merchandise
- Any fan-fiction besides for The Cursed Child