Despite still being developed, HTML5 is already in use on more than 1% of the world's websites.
Netcraft's December Web Server Survey found the HTML5 DOCTYPE on 1.06% of homepages. An additional 0.05% of sites made use of new HTML5 features without explicitly declaring the correct DOCTYPE.
HTML5 is the next major revision of the HTML standard, and looks set to supersede HTML 4.01 and XHTML 1.1 with its support for video and audio playback. Microsoft has recently shifted its strategy on Silverlight as a cross-platform solution and now wants to implement standards-based HTML5 really, really, really well in Internet Explorer 9.
Earlier this year, Google also announced that it was shifting effort towards bringing their now-deprecated Gears API capabilities into HTML5 standards. Google's Chrome browser uses the same WebKit rendering engine as Apple browsers, with Apple boasting support for HTML5 on each of its new mobile devices and Macs.
Desktop browser support for the full set of new features in HTML5 is still rather patchy, although it should be noted that the HTML5 specification is still a working draft and subject to change. Many web designers will be reluctant to use the newest features just yet, as a significant fraction of their visitors will be unable to enjoy the content as it was intended. This is clearly demonstrated by the rarity of HTML5 elements such as canvas (appearing directly on only 0.012% of homepages), video (0.011%) and audio (0.003%).