The idea is by no means new. localStorage is old. So is the AppCache specifications that actually allows an application to go 'offline'. Gmail did this as early as 2009: http://googlecode.blogspot.sg/2009/04/gmail-for-mobile-html5...

Unfortunately it's not as easy as it looks. I found that out when doing research for a weekend project. Here's an excellent A List Apart article detailing all the gotchas you can face while working with it: http://www.alistapart.com/articles/application-cache-is-a-do.... The Financial Times Labs have an interesting article talking about an approach to deploying an offline app that uses the AppCache as little as possible: http://labs.ft.com/2012/08/basic-offline-html5-web-app/

And of course you can find information about it in places like Dive into HTML5 and MDN, by simply searching for them