Is there any way to have some persistent data (like an XML file or so) apart from cookies at client side? I know that's not possible with JavaScript directly. But any other way like Applets or something??
If cookies won't suffice, and you have to support older browsers, then LawnChair might help: http://brian.io/lawnchair/plugins/
It's like a "Couch" but smaller, get it? Took me a few minutes ;).
And, it does work for desktop browsers, despite the blurb on the site about mobile-webkit: https://github.com/brianleroux/lawnchair/blob/master/README.md.
(But, it uses cookies as the adapter to store data for the fallback on those older browsers, most such libraries probably will, if you're not allowed to use cookies for some reason, these types of libraries might not fit the bill.)