10) Costume Callbacks
While Waller’s voiceover introduces Harley, we briefly see the Joker and Ms. Quinn dancing through a black void – he’s in a tuxedo, while she’s in her classic jester outfit (from Batman: The Animated Series).
Though it’s cool enough that Harley’s traditional costume appears in some form in the movie (Harley also takes it out of her trunk later on), this scene is actually a much more specific easter egg than that. The sequence is perfect recreation of the cover to 1999’s Batman: Harley Quinn from artist Alex Ross. The image has since become an icon of the Clown Prince and Princess’ relationship.
In fact, most of the outfits the Joker wears in the film call back to the comics. His white suit with purple shirt is a nod to The Dark Knight Returns and his brown waistcoat is from Joker.
Speaking of which…