In my testing, you don’t need to fully embrace HTML5 markup to take advantage of the “offline” functionality, you simply need to add the attribute and related files to your existing website/page. Any modern browser that supports HTML5 should automatically recognize the offline content and use it when appropriate, unfortunately no version of MSIE yet supports this.
<html manifest="/offline.appcache">
In that file, you must then specify the offline behavior, something like this is a good start:
CACHE MANIFEST
#This is to provide minimal HTML5 offline capabilities
#MIME mapping must be 'appcache=text/cache-manifest'
#Reference to this file is per page, you can have different ones in an app.
#Common image files and css may be 'cached'
CACHE:
/index.html
/css/offline.css
NETWORK:
*
FALLBACK:
/ /offline.html
On the server side, you’ll have to serve up that file with the appropriate MIME type (text/cache-manifest, for ApacheHTTPD you simply need to add one line to httpd.conf:
AddType text/cache-manifest .appcache
REFERENCES: