Get screen density and viewport width, useful for Analytics

responsive-context.js

// get device pixel ratio in dppx
// https://github.com/ryanve/res/blob/master/src/index.js
var screen_density =
  typeof window == 'undefined'
    ? 0
    : +window.devicePixelRatio ||
      Math.sqrt(screen.deviceXDPI * screen.deviceYDPI) / 96 ||
      0
// keep only 3 decimals: http://jsfiddle.net/AsRqx/
screen_density = +(Math.round(screen_density + 'e+3') + 'e-3')

// get viewport width
// http://stackoverflow.com/a/8876069/717195
var viewport_width = Math.max(
  document.documentElement.clientWidth,
  window.innerWidth || 0,
)