Untitled

=== index.html ===

<!doctype html>
<html manifest="test.appcache">
<head>
	<meta charset="utf-8">
	<title>Connectivity Test</title>
	<script src="script.js"></script>
</head>

<body>
	<h1>Let's test this connection!</h1>
	<button onclick="go()">Do it!</button>
</body>
</html>

=== script.js ===

function testOnline(fn) {
	var script = document.createElement('script');
	script.src = 'online.js';

	window.setOnline = function(online) {
		document.body.removeChild(script);
		fn(online);
	}

	document.body.appendChild(script);
}

connectivity = function(online) {
	online ? alert("Online") : alert("Offline");
}

function go() {
	testOnline(connectivity);
}

=== online.js ===

setOnline(true);

=== offline.js ===

setOnline(false);

=== test.appcache ===

CACHE MANIFEST

# Version: 1

CACHE:
index.html
script.js

NETWORK:
*

FALLBACK:
online.js offline.js

=== .htaccess ===

AddType text/cache-manifest .appcache

<IfModule mod_expires.c>
  ExpiresActive on
  ExpiresDefault                          "access plus 1 month"

  ExpiresByType text/cache-manifest       "access plus 0 seconds"
  ExpiresByType text/html                 "access plus 0 seconds"
  ExpiresByType application/json          "access plus 0 seconds"
  ExpiresByType text/css                  "access plus 0 seconds"
  ExpiresByType application/javascript    "access plus 0 seconds"

 <IfModule mod_headers.c>
  Header append Cache-Control "public"
 </IfModule>

</IfModule>

<IfModule mod_headers.c>
  Header unset ETag
</IfModule>

FileETag None