HTML5

In HTML5, you won't have to worry about installing a plugin to listen to a song or watch a video on a website. This is a good thing for platforms that doesn't support Flash (e.g., iPhone and iPad) or have some problems with it (e.g., Linux). It will also be a boon to smartphones where supporting Flash is problematic.

HTML5 is popular because of its features like media playback by tags and offline storage.
With HTML4, sites have to go for Flash or Silverlight to show a video or play music. HTML5 lets sites directly embed media with the simple HTML tags "video" and "audio" -- no plugins are required.

With this, we will be able to create files in Google Docs or draft e-mails when away from an Internet connection. These changes would be automatically synchronised  the next time we go online.

HTML5 also adds new interactivity features, like drag-and-drop, that have already found their way into Gmail.

Safari, Google Chrome and Firefox 3.6 all support at least some elements of HTML5.  And many Google products already use some features.

Practical difficulties restrict HTML5 to dominate and kill Flash & Silverlight..
HTML5 has a long way to go for establishing it's strong presence..

For more: http://www.theregister.co.uk/2009/08/28/html_5_dream/