HTML5 is a markup language for structuring and presenting content for the World Wide Web, and is a core technology of the Internet originally proposed by Opera Software. It is the fifth revision of the HTML standard (created in 1990 and standardized as HTML4 as of 1997) and, as of May 2012, is still under development. Its core aims have been to improve the language with support for the latest multimedia while keeping it easily readable by humans and consistently understood by computers and devices (web browsers, parsers, etc.). HTML5 is intended to subsume not only HTML 4, but XHTML 1 and DOM Level 2 HTML as well.
Thanks, Wikipedia!
According to different reports, HTML will not be ready until next year. It won't support some current HTML tags like <center> or <font>. Those should be replaced with text-align:center; and font-family, color, etc.
Some demos you can try: html5demos.com - Note that videos require almost no buffering at all!
Some HTML5 games demos: html5games.com - Most of them are still "raw" though, but hey, they're not Flash-based!
Is your site HTML5-ready yet?