Two standards bodies with different aims, working on different timescales, cloud the future of HTML. Here's a guide for those who test, deploy, and support browsers and systems.
The World Wide Web Consortium, W3C, which defines many of the standards that make up the Web, does not operate on Internet time. After standardizing XHTML 1.1 in 2001, the body took 5 years to produce 8 drafts toward the definition of XHTML 2.0. Not nearly agile enough.
Engineers from companies making Web browsers (Apple, Mozilla, and Opera) got together on a faster track and in 2003 the Web Hypertext Application Technology Working Group (WHATWG) was born. They started working toward the definition of HTML5, intended as the evolution of HTML that would support Web applications in a robust way. They had an advantage over the W3C in that they could base their work on actual implementations in living browsers.
In 2007 the W3C endorsed the WHATWG's work on HTML5 by adopting it as the basis for the W3C-sanctioned working draft of HTML5. By 2009, the W3C had abandoned work on XHTML 2.0 in favor of HTML5.
The WHATWG is now pushing to drop the "5" (wonder what will become of that spiffy logo?) and stress that what they are working is on simply "HTML" -- a collection of evolving features rather than a completed specification. HTML itself becomes a "living standard," where implementations trump standards.
The result of this fluidity: the only reliable way to implement HTML going forward will be to target specific browsers, and specific versions of specific browsers, not some particular version of a standard. Which of the many features of HTML5 you implement, for internal or external use, will depend on what browsers and versions you need (external) or choose (internal) to support.
Here are specific suggestions to keep your HTML development and browser deployment and support on the right track in this fast-changing environment.
Many more details and links can be found in this Computerworld article, which will repay your close reading.