The World Wide Web Consortium (W3C) has published a public working draft of HTML 5 -- the first major update to HTML in 10 years.
HTML 5 factors in the shift from Web sites being just a collection of static pages to communities that leverage upon rich-media applications and online collaboration. There are APIs for drawing two-dimensional graphics, managing the storing of data on the client-side, and other features such as embedding and controlling multimedia.
HTML 5 is designed to inject more consistency into the ways vendors and end users have gone about building today's generation of sites. According to the W3C, HTML 5 will: "Improve interoperability and reduce software costs by giving precise rules not only about how to handle all correct HTML documents but also how to recover errors."
The work is spearheaded by W3C's HTML Working Group, which was founded in March of 2007. You might be interested to know that its members include Google, IBM, Microsoft, Mozilla, and Nokia.