Page Visibility API - http://code.kojilabs.com/page-visibility-api/

pageVisibility.js

/**
 * The Page Visibility API adds two new properties to the document object.
 * 
 *      document.hidden
 *      document.visibilityState
 *
 * The hidden property is a boolean representing the current visibility of the page.
 * 
 * The visibilityState property is a string that gives more information about the current page state. This property has four possible values:
 *
 *      hidden – not visible on any screen
 *      visible – visible to the user
 *      prerender – the page loaded off-screen and is not visible
 *      unloaded – the user is navigating away from the page
 */

function getPrefixed() {
  // Check for support of the un-prefixed property.
  if ('hidden' in document) {
    // No prefix needed, return null.
    return 'hidden';
  }

  // Create an array of the possible prefixes.
  var prefixes = ['moz', 'ms', 'o', 'webkit'];

  // Loop through each prefix to see if it is supported.
  for (var i = 0; i < prefixes.length; i++) {
    var testPrefix = prefixes[i] + 'Hidden';
    if (testPrefix in document) {
      // Prefix is supported!
      // Return the current prefixed property name.
      return testPrefix;
    }
  }

  // The API must not be supported in this browser, return null.
  return null;
}