init.js

// Variables
var hasTouch = false,
  ua = navigator.userAgent,
  docEl = document.documentElement;

// Clear html classes and add "js" class
docEl.className = "";
docEl.className += " js";

// Determine if the device supports touch
if (("ontouchstart" in window) || window.DocumentTouch && document instanceof DocumentTouch) {
  docEl.className += " touch";
  hasTouch = true;
}

// Using UA detection is the only way to solve font-face problems on
// devices which report that they support it, but really don’t.
if (ua.match(/(Android (2.0|2.1))|(Symbian)|(Opera (Mini|Mobi))|(w(eb)?OSBrowser)|(UCWEB)|(Windows Phone)|(XBLWP)|(ZuneWP)/)) {
  docEl.className += " nofontface";
}

// Test element
var testEl = …;

// The touchstart event fires before the mousedown event, and therefore
// we can wipe the mousedown event handler before it’s ever executed
// http://www.quirksmode.org/blog/archives/2010/02/do_we_need_touc.html
testEl.onmousedown = function () {
  // initialize mouse interface
}
testEl.ontouchstart = function () {
  testEl.onmousedown = null;
  // initialize touch interface
}