HTML is the language that tells your browser what to do, whether that's showing a LOLcat or running a browser-based application, and HTML5 is its latest incarnation. It's designed to handle the way the Web works now and in the future, which means support for Web applications and multimedia, and your browser already supports some of its key features.
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Still none the wiser? Don't worry: here's your quick guide to what it is, why it matters and why people are getting excited about it.
1. It could replace Flash and Silverlight in a lot of sites…
Lots of websites use Flash or Silverlight technology to deliver video, and that's something HTML5 is perfectly capable of doing - without requiring the installation of a plugin. Other sites use Flash or Silverlight to deliver animation. HTML5 can do that too - without requiring the installation of a plugin.
2. …but it won't kill Flash or Silverlight
There's a lot more to Flash and Silverlight than mere video playback. Replicating some of the more interesting games, interfaces and Rich Internet Applications is a whole different ballgame. Remember, too, that Flash and Silverlight aren't carved in stone or dependent on browser support: if Adobe or Microsoft dream up a cool new feature they can build it, stick it in their plugin and update all their users almost immediately.
There's also the issue of hardware video acceleration, which the latest Flash Player - on Windows - takes advantage of: in tests by Jan Ozer Flash was less of a hardware hog than HTML5.
There's another very good reason why the Web won't entirely switch to HTML 5 audio and video: DRM.
3. It doesn't do DRM
Publishing content in unprotected, easily duplicated formats won't appeal to some of the bigger media firms, so they'll stick with what they're currently using.
4. It's good news for Web app users
Some of the most important bits of HTML5 are under the hood where they make coding Web applications simpler and the apps themselves more stable - which means better performance and fewer crashes. It also provides a mechanism for offline data storage, blurring the boundaries between web apps and desktop ones.
5. It's killed Google Gears…
Gears was Google's technology for making its web applications work offline, but the search giant has decided that HTML5 is a better solution to the same issue. As a result, it's stopped developing Google Gears (although it's still supporting the technology until HTML5 can do every single thing Gears does).
6. …but it won't kill Photoshop
HTML5 is great for browser-based applications, but trying to replicate something as complicated - and as performance-dependent - as Photoshop would be madness. Madness!
7. It knows where you live
One of the most interesting bits in HTML5 is its support for Geolocation, which works out where you are (based on your IP address, or GPS if your hardware has a GPS chip) provided you give it the go-ahead. That opens up some interesting possibilities such as location-aware search results, or Big Brother-style monitoring by the boss.
8. You can use it right now…
All the major browser makers are baking HTML5 into their browsers like chocolate chips in a cookie, so for example the welcome screen in Apple's Safari is an HTML5 effort. You'll find lots of HTML5 demos at the cunningly named HTML5 Demos website.
9. …but it's all over the place
As you probably noticed on HTML5 Demos, many of the demos only work on a few browsers. That's because different browsers have different levels of support, and differing approaches to common elements - so for example while Safari, Chrome and Firefox all support HTML5 video, Firefox only works with the Ogg Theora codec while the others prefer H.264. Internet Explorer doesn't support either.
10. It's evolution, not revolution
Introducing HTML5 to the Internet isn't a big, dramatic event where some Web guru flicks a giant lever and we all switch over to a new technology. It's much more subtle than that. With each new browser version more of HTML5 becomes supported, and as that support improves websites will start to take advantage of more HTML5 features.