For the last couple of months the general thinking has been mobile first. The justification that most architects give is the mobile is easier to implement first and many of the features that provide translate directly to desktop.
This morning I read a tweet where the author suggested that the platform for the first. And it is impossible to infer any sort of details from 140 character tweet.
What I realize however what makes doing a particular task first is of utmost importance. The team that performs mobile first is typically weak in mobile. The team that implements platform first is typically weak in platform.