HTML goes back to 1993 when it was first published as internet draft. It evolved through such important milestones as 2.0, 3.2, and 4.0. However, there were few shortcomings in HTML that people saw as show stoppers for the HTML further development. However, a group of people decided to take it to the next level and created Web Hypertext Application Working Group (WHATWG) in 2004. This was the year when HTML5 specification was published for the first time.
The Web Hypertext Application Working Group got a boost when W3C started to look at HTML 5 in 2006 and published its own draft of HTML5 in 2008. This step prompted all of the Browser supporting organization to implement features published in HTML5 draft and many new browsers are coming up with HTML5 enabled. Moreover, there is a feedback loop that exists between browser producers and HTML5 users that feed on each other for new features and implementations of HTML5.
There are now two important milestones that are set for HTML5. First, it is 2012 when candidate recommendation will be published. Second, it is 2022 when proposed recommendation will be published.
It is important to note that HTML5 is not supported by IE 6.0 and any support by future versions of IE, FireFox or Chrome will be gradual.