HTML5 is the latest and greatest update to the ancient markup language. It has many drastic changes that could revolutionize the web. It is the future of many platforms and you can rest assured, the next version of SharePoint will use it!

In order for your SharePoint site to support HTML5, you need to edit your Master Page, and do the following...
<ul>
	<li>Change the DOCTYPE tag to this: &lt;!DOCTYPE html&gt;</li>
	<li>Remove the following tag: &lt;meta http-equiv="X-UA-Compatible" content="IE=8"&gt;</li>
</ul>
After doing this, you can put HTML5 specific markup tags (can in your site.  For an example of a SharePoint site using HTML5, check out the following link...

<a href="http://www.sharepointisawesome.com/">http://www.sharepointisawesome.com</a>

If you view the site with a browser that supports HTML5 (IE9+, Chrome, etc), then you should see a video.  However, if you open the site in a non-HTML5 browser or put IE9 into IE8 compatibility mode, the video disappears.

Now that you know how easy it is to make SharePoint HTML5-friendly, go download the <a href="http://visualstudiogallery.msdn.microsoft.com/d771cbc8-d60a-40b0-a1d8-f19fc393127d/">HTML5 Intellisense for Visual Studio 2008-2010</a> and start coding!