I was watching the ending of Return of the Joker where Terry fights the Joker one-on-one. Towards the end of the fight Terry says something that caught my attention:
So you fell in a tank of acid, got your skin bleached, then decided to become a super villain. What, you couldn't get work as a rodeo clown?
For reference:at 2:43
Which is odd. I'm not familiar with Batman: The Animated Series ever dedicating itself to one specific Joker origin, let alone The Killing Joke. In fact it runs kind of counter-intuitive to the peek at the Joker's origin from Mask of the Phantasm where
The Joker was an ex-mobster working with Sal Valestra.
Did The Animated Series ever confirm that Joker became a villain after falling in a vat of acid?