Markup languages are a dime a dozen, and knowing one means knowing any other language is a breeze. I wasn't able to think of more than a few different markup languages off the top of my head. They aren't complicated, and they all work in nearly the same way. The nuances between them are easily found online, so what's the value in memorizing them? The frequency of new languages and syntax doesn't bode well for becoming too attached to any one of them.