You don't always need to know which way up a device is being held. You just need to be able to access the values for height and width. This seems to work:

gistfile1.txt

var devicePixelRatio = window.devicePixelRatio || 1,
    ww = devicePixelRatio * (window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth),
    wh = devicePixelRatio * (window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight);


// width, height
var landscape = [Math.max(ww, wh), Math.min(ww, wh)];
var portrait = [Math.min(ww, wh), Math.max(ww ,wh)];