HTML5, if 'fully' supported by browsers (like Safari and Chrome), supports a bunch of new features that make previously impossible stuff easily doable - the major one being offline storage. 
e.g: GMail can store your inbox in your browser's offline storage cache, so you can continue to use the site and send/read emails just like you do with Outlook or other mail clients today.  When you reconnect to the web, it'll sync up and send the differences.