All evidence seems to point to the design of Ledger's clown mask being modeled directly on a scene from "The Joker Is Wild," an episode of the Batman TV series that also marked the first appearance of the Joker on the show. In the episode, Adam West's Batman tracks Romero's clown prince of crime to the Gotham opera company, where he's secretly performing an aria from Pagliacci, because back then that was as close as the Joker could get to being an actual killer clown (look it up). As you can see, the similarities between Romero's Pagliacci costume and Ledger's mask are undeniable.
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Good luck unseeing that anytime soon.
And if anyone out there is wondering why Nolan even bothered referencing an old comedy series that was campier than a Boy Scout jamboree: While it's true that The Dark Knight was basically a superhero version of Michael Mann's Heat, the 1960s series is what has kept Batman alive in the public consciousness for decades, so despite its silliness, it still deserves all the respect it can get. The fact that the man who first brought the Joker to the big screen didn't respect the character enough to even shave off his mustache for the role should really be beside the point.