Visibility API

gistfile1.js

//http://www.w3.org/TR/2011/WD-page-visibility-20110602/
(function () {
	"use strict";

	//no need to shim if it's already there
	if ( 'hidden' in document && 'onvisibilitychange' in document ) {
		return;
	}

	//fail silently if the browser sux
	if (
		!Array.prototype.forEach   ||
		!Object.defineProperty     ||
		!document.addEventListener ||
		!window.addEventListener
	) {
		return;
	}

	//check to see if there's a native implementation
	var prefixes = [ 'webkit', 'moz', 'o', 'ms' ], nativeName = '';
	prefixes.some(function ( prefix ) {
		if ( (prefix + 'Hidden') in document ) {
			nativeName = prefix;
			return true;
		}
	});

	var hiddenGetter;
	if ( nativeName ) {
		hiddenGetter = function () {
			return document[ nativeName + 'Hidden' ];
		};
	} else {
		hiddenGetter = function () {
			return hidden;
		};
	}
	//corresponding values to document.hidden
	var visibilityStates = {
		'true' : 'hidden',
		'false' : 'visible'
	};
	var stateGetter = function () {
		return visibilityStates[ document.hidden ];
	};

	//will be called whenever the visibility has been changed
	var visibilityChange = function () {
		document.hidden = hiddenGetter();
		document.visibilityState = stateGetter();

		//create the visibilitychange event
		var evt = document.createEvent( 'Event' );
		evt.initEvent( 'visibilitychange', true, true );
		//you're FIRED!
		document.dispatchEvent( evt );

		//check for document.onvisibilitychange and call it if it exists
		var vischange = 'onvisibilitychange'; //it's really long to write...
		if ( document[vischange] && document[vischange].call ) {
			document[ vischange ].call( document, evt );
		}
	},
		hidden = false;

	//add the event listeners to tap into nativeName functionality
	if ( nativeName ) {
		document.addEventListener(
			nativeName + 'visibilitychange',
			visibilityChange,
		false );
	} else {
		//the window.focus and window.blur events are, well, close enough
		window.addEventListener( 'focus', function () {
			hidden = false;
			visibilityChange();
		}, false );
		window.addEventListener( 'blur', function () {
			hidden = true;
			visibilityChange();
		}, false );
	}

	//add document.hidden, document.visibilityState and
	// document.onvisibilitychange
	Object.defineProperties(document, {
		hidden : {
			value : hiddenGetter(),
			writable : true,
			configurable : true
		},
		visibilityState : {
			value : stateGetter(),
			writable : true,
			configurable : true
		},
		onvisibilitychange : {
			value : null,
			writable : true,
			configurable : true
		}
	});
}());

Test window.addEventListener as well.

@Raynos Thanks, it managed to slip through. Also added a link to the w3c page.

Note:

1. Google Closure Compiler can't parse your code due 'native' word
2. addEventListener without 3rd parameter didn't work in FF 3.6 and lower due to bug and it can't be shimed

@termi1uc1 Thanks for those, forgot about the third parameter. Fixed.