They can be a joke, a gag, a dream sequence, or even a "what if" scenario (such as DC's Elseworlds). Avengers/JLA is in canon, but most Marvel/DC crossovers are non-canon. They include those where the characters live in alternate universes, as well as those where they share the same Earth.

Has Marvel ever had a DC crossover?

Superman vs The Amazing Spider-Man was the first DC/Marvel crossover ever published. DC and Marvel had previously collaborated on a treasury edition of The Wizard of Oz, but this is the first time characters owned by both companies were used together.

Do Marvel and DC ever meet?

A crossover between Marvel and DC has actually already happened four times in the comics. The first time with Superman and Spider-Man in the aptly named "Superman vs.