Before tumblr, I used to think that headcannons were small theories based on evidence and subtle things found in the cannon. Making it plausible for them to coexist with the cannon.
What tumblr has taught me, however, is that headcannons are just wild and unevidenced claims based in absolutely nothing and sometimes outright ignoring the cannon to make it pluasible. Which seems more like it would be an AU than anything.
What tumblr has also taught me is that people will fight you to the fucking death on their headcannons, swearing up and down that they’re not only plaisible, BUT THE REAL CANNON.
Which is weird to me. like, chill they are a fictional character.