HTML 5 represents a game changer in web standards since the current (4.01) specification. HTML 5 is not merely intended to present content in a web browser, but its goal is to bring the web as a full-fledged application platform, where video, sound, images, and animations are all standardized. Most promising features of HTML5 are its offline data storage capabilities and incorporation of Flash (a missing capability in iPad).


Offline Gmail  feature is definitely a hit with web interface users,  but unfortunately, Gmail for iPhone has no offline capabilities. Therefore, Google is persistently working with HTML5  to solve this shortcoming. Vic Gundotra, Google’s VP of engineering, recently demonstrated a new, private version of Gmail that uses the offline storage tools coming in HTML 5. These tools offer offline access to Gmail through the service’s mobile interface on both the iPhone and Android platforms.

Here is a short video of demo



HTML 5 is still in its infancy but as for Gmail's webapp for mobile, it sure looks promising. Also, the best part for developers is that there’s no need to write a separate app for every mobile platform — the same interface will work on any mobile device as long as the browser supports HTML 5.

Developer's are also rigorously testing video capabilities on HTML 5 browsers, since iPad doesn't support Flash. There many things that Flash can be used for, but what about games? Well, HTML5 might be a more powerful game engine than most people think. Here is short video shown by Google's engineers to show what exactly is possible with HTML5 in the browser. Google Web Toolkit team created an HTML5 port of the classic first-person shooter game Quake II.



 
The graphic capabilities are enormous for a game that is actually run on a browser. However, at the moment, because HTML 5 is still being developed, browser support is primarily limited to modern browsers like Chrome, Safari and Firefox.Whatever may be the case, I believe this is a huge leap from a document markup and plugins to be able to do much more in the document itself, straight from the browser.