Here's and interesting, and somewhat surprising, one.
I looked up "dunno" (the contraction of "don't know") in the online Merriam-Webster, and it was absent. I admit to some surprise, as the expression is in common use in the USA. I looked in my old Funk & Wagnalls, and again if failed to appear. More surprise.
Then I looked in the online Oxford, where its use is described as informal. In my old Oxford, it is down as colloquial. Does this mean that its use has become more acceptable?