HTML5 is a specification being formalized by the Web Hypertext Application Technology Working Group (WHATWG) that defines concrete language syntax for an API that can describe documents and applications. The WHATWG specification incorporates both the existing HTML 4.01 and XHTML 1.0 features, and also introduces new items, including:
new layout elements
programming changes to the Document Object Model (DOM)
updated Web Forms
server-sent DOM events
dynamic graphics capabilities
HTML5 is the next major revision of the HTML standard superseding HTML 4.01, XHTML 1.0, and XHTML 1.1. HTML5 is a standard for structuring and presenting content on the World Wide Web.
HTML5 is a cooperation between the World Wide Web Consortium (W3C) and the Web Hypertext Application Technology Working Group (WHATWG).
The new standard incorporates features like video playback and drag-and-drop that have been previously dependent on third-party browser plug-ins such as Adobe Flash, Microsoft Silverlight, and Google Gears.
Browser Support:
The latest versions of Apple Safari, Google Chrome, Mozilla Firefox, and Opera all support many HTML5 features and Internet Explorer 9.0 will also have support for some HTML5 functionality.
The mobile web browsers that come pre-installed on iPhones, iPads, and Android phones all have excellent support for HTML5.
New Features:
HTML5 introduces a number of new elements and attributes that helps in building a modern websites. Following are great features introduced in HTML5.
New Semantic Elements: These are like<code>, <header>, <footer>, and <section>.
Forms 2.0: Improvements to HTML web forms where new attributes have been introduced for <input> tag.
Persistent Local Storage: To achieve without resorting to third-party plugins.
WebSocket : A a next-generation bidirectional communication technology for web applications.
Server-Sent Events: HTML5 introduces events which flow from web server to the web browsers and they are called Server-Sent Events (SSE).
Canvas: This supports a two-dimensional drawing surface that you can program with JavaScript.
Audio & Video: You can embed audio or video on your web pages without resorting to third-party plugins.
Geolocation: Now visitors can choose to share their physical location with your web application.
Microdata: This lets you create your own vocabularies beyond HTML5 and extend your web pages with custom semantics.
Drag and drop: Drag and drop the items from one location to another location on a the same webpage.
Backward Compatibility
HTML5 is designed, as much as possible, to be backward compatible with existing web browsers. New features build on existing features and allow you to provide fallback content for older browsers.
The Need for HTML5
Apple, Mozilla, and Opera became increasingly concerned about the W3C's direction (or lack of direction) with XHTML, their lack of interest in HTML, and their perceived disregard for the needs of web application developers. In response, these organizations took it upon themselves to address these concerns.
Markup for documents on the World Wide Web has always been some incarnation of HTML. Although it was originally designed as a language for semantically describing scientific documents, HTML was adopted for general use and was rapidly extended during the 1990's. It's now used to describe most documents transmitted across the web.
HTML worked well for publishing static web pages. But many modern web documents aren't individual static pages at all; instead, they're partial pages or one page among many that, collectively, compose a web application. The current HTML specification inadequately addresses the entire area of web applications (session-oriented conversations between web clients and web server components). The WHATWG specification is an attempt to correct this situation and, at the same time, update the HTML specifications to address other issues that have annoyed web developers over the last few years.