HTML5 is an under developing version of HTML4.01, XHTML1, and DOM Level 2 that is expected to come in the market by the end of 2010, according to World Wide Web Consortium (W3C) which is currently co-working on it with Web Hypertext Application Technology Working Group (WHATWG).
HTML5 is the most advanced upcoming version in which several new features like MIME type and protocol handler registration, microdata, Geolocation, local Sql database, web SQL database, Indexed hierarchical key-value store (formely WebSimple DB), Indexed Database API have been included that are necessary for web applications. Several other restrictions have been removed and some of APIs have been modified from the previous versions.
HTML5 is a simple HyperText Mark Up Language that can be written in both HTML (HTML5) and XML (XHTML5) language. In the latest version of HTML, several APIs like Canvas element, Timed media playback, Offline storage database, document editing, drag-and-drop, cross-document messaging, browse history management have been added that form the basis of the Web architecture.
WHATWG had originally proposed HTML5 in June 2004 which is accepted by W3C in 2007. It is still under developing stage and some of its components may likely to hit the market in late 2010 while complete and comprehensive HTML5 language with full specifications can take over a decade, as per W3C. Ian Hickson ? the author and maintainer of the Acid2 and Acid3 tests, and the Web Applications 1.0 who is now in Google is the editor of HTML5.
HTML5 has first time defined the DOM Level 0 documentation which was never defined in the earlier versions. HTML5 is not based on the SGML (Standard Generalized Markup Language) specification, whereas all HTML versions up to 4.01 were.
HTML5 like previous versions has the features of Markup but in different ways. The Syntax of HTML5 is no longer based on SGML but has been designed in such a way that it supports all its previous versions. HTML5 has introduced a new document type declaration <!DOCTYPE html> line that looks like a SGML syntax. <!DOCTYPE html> informs the browser about the version and type of the documents that make it compatible in rendering the file.
Several new elements and attributes have been included in HTML5 which can be very useful in making modern websites. Some of them are semantic replacements for common uses of generic block (<div>) and inline (<span>) elements such as <nav> (website navigation block) and <footer>. Other elements provide new functionality through a standardized interface like multimedia elements <audio> and <video> etc.
HTML5 also incorporates Web Forms 2.0, another WHATWG specification.
Here is the list of some new APIs been included in HTML5:
Timed media playback
Note: Some of the new features are part of HTML5 and some are maintained in separate specifications.