So Fox News and Breitbart are mainstream media?
Fox is the most watched TV channel in the United States — hard to argue they're not an essential part of the current political environment. And Breitbart is certainly more prominent and influential than a niche site like The Verge. The latter's editors don't get White House jobs.
Its worth nothing that although there are no strict definitions of what counts as mainstream vs independent, the populist wings of both parties loosely group them in the same buckets.
A lifetime ago, I used to consider myself populist-left and since then there hasn't been much if any difference in what I consider as mainstream.