Here’s the deal. Your grandma can now create a viewport. And so can the kid next door. These may not be ‘proper’ browsers, and they may not (yet) be fully interactive, but they can load a pretty sophisticated web page. A year from now, the most popular ‘browser’ may just be be the embedded web view full of ‘related’ links in a Stephen King iBooks bestseller.
Basing breakpoints on popular viewport dimensions is clearly a foolish pursuit.