Offline Web applications sounds like an oxymoron, but it's been possible for several years via the support for Application Caches in modern browsers. This includes the mobile iOS and Android browsers as well as desktop browsers like Firefox and Chrome. With support for going offline in Internet Explorer 10, this feature is ready for you to start taking seriously.
What kind of applications can you actually write if you're "offline" and not able to make Ajax requests to your server? Don't worry, browsers are starting to include rich database engines that make it possible to store and query data when you don't have access to your server. Later, when back online, you can synchronize that locally stored data with the server.
This course guides you through the technologies that make this possible.