See https://twitter.com/thijs/status/986866734245335042

tilt.js

// On iPhone and iPod Touch we add a nice dynamic 3D parallax effect by 
// updating the perspective origin depending on device tilt.
if (/iPhone|iPod/.test(navigator.platform) && 'ondevicemotion' in window) {
  ;(function() {
    var value,
      element = $('div.doors ul').get(0),
      values = {x: [0,0,0], y: [0,0,0]},  // Keep the 3 most recent values.
      index = 0,
      choices = {  // Lookup table to normalize based on device orientation.
        x: {'0': ['x', -1], '90': ['y', 1], '180': ['x', 1], '-90': ['y', -1]},
        y: {'0': ['y', 1], '90': ['x', 1], '180': ['y', -1], '-90': ['x', -1]}
      }
    
    // The devicemotion event is triggered at regular intervals, so this code 
    // is optimized to be fast rather than elegant.
    window.ondevicemotion = function(event) {
      // Get the device tilt and normalize its value to match the current 
      // orientation of the device.
      values.x[index] = event.accelerationIncludingGravity[choices.x[window.orientation][0]] * choices.x[window.orientation][1]
      values.y[index] = event.accelerationIncludingGravity[choices.y[window.orientation][0]] * choices.y[window.orientation][1]
      index++
      if (index == 3)
        index = 0
      // Average the 3 most recent values and create the css value string.
      value = (50 + ((values.x[0] + values.x[1] + values.x[2]) / 3 * 10)) + '%' + (50 + ((values.y[0] + values.y[1] + values.y[2]) / 3 * 10)) + '%'
      // Update the perspective origin based on the device tilt around the x
      // and y axis.
      element.style.perspectiveOrigin = value
      element.style.webkitPerspectiveOrigin = value
    }
  })()
}