Re: The Joker's background?
Well, the chemical vat is. It was the origin given to him by Kane/Finger and he was always someone with permanet white skin, green hair and permanet smile. And then after the Crisis of Infinitive Earth the almost same origins were restated in The Killing Joke and then repeated/confirmed in O'Neil's 'Images' and in 2004's Gotham Nights #54. There were some other suggestions in 2000s, but they were never confirmed or definite like in the above mentioned comics and they never contradicted the chemical accident