What is HTML 5 ??

HTML5 is the latest version of HTML and XHTML. The HTML standard defines a single language that can be written in HTML and XML. It attempts to solve issues found in previous iterations of HTML and addresses the needs of Web Applications, an area previously not adequately covered by HTML.
HTML5 differences from HTML4

What Is HTML5
HTML5 is a language for structuring and presenting content for the World Wide Web, and is a core technology of the Internet originally proposed by Opera Software. It is the fifth revision of the HTML standard (created in 1990 and standardized as HTML4 as of 1997) and as of December 2011 is still under development. Its core aims have been to improve the language with support for the latest multimedia while keeping it easily readable by humans and consistently understood by computers and devices (web browsers, parsers, etc.). HTML5 is intended to subsume not only HTML 4, but XHTML 1 and DOM Level 2 HTML as well.



Following its immediate predecessors HTML 4.01 and XHTML 1.1, HTML5 is a response to the observation that the HTML and XHTML in common use on the World Wide Web are a mixture of features introduced by various specifications, along with those introduced by software products such as web browsers, those established by common practice, and the many syntax errors in existing web documents. It is also an attempt to define a single markup language that can be written in either HTML or XHTML syntax. It includes detailed processing models to encourage more interoperable implementations; it extends, improves and rationalises the markup available for documents, and introduces markup and application programming interfaces (APIs) for complex web applications. For the same reasons, HTML5 is also a potential candidate for cross-platform mobile applications. Many features of HTML5 have been built with the consideration of being able to run on low-powered devices such as smartphones and tablets. In December 2011 research firm Strategy Analytics forecast sales of HTML5 compatible phones will top 1 billion in 2013.

In particular, HTML5 adds many new syntactical features. These include the <video>, <audio>, <header> and <canvas> elements, as well as the integration of SVG content that replaces the uses of generic <object> tags. These features are designed to make it easy to include and handle multimedia and graphical content on the web without having to resort to proprietary plugins and APIs. Other new elements, such as <section>, <article>, <header> and <nav>, are designed to enrich the semantic content of documents. New attributes have been introduced for the same purpose, while some elements and attributes have been removed. Some elements, such as <a>, <cite> and <menu> have been changed, redefined or standardized. The APIs and document object model (DOM) are no longer afterthoughts, but are fundamental parts of the HTML5 specification. HTML5 also defines in some detail the required processing for invalid documents so that syntax errors will be treated uniformly by all conforming browsers and other user agents.

XHTML5

XHTML5 is the XML serialization of HTML5. XML documents must be served with an XML Internet media type such as application/xhtml+xml or application/xml.[48] XHTML5 requires XML's strict, well-formed syntax. The choice between HTML5 and XHTML5 boils down to the choice of a MIME/content type: the media type one chooses determines what type of document should be used. In XHTML5 the HTML5 doctype html is optional and may simply be omitted. HTML that has been written to conform to both the HTML and XHTML specifications—and which will therefore produce the same DOM tree whether parsed as HTML or XML—is termed "polyglot markup".

Error handling

An HTML5 (text/html) browser will be flexible in handling incorrect syntax. HTML5 is designed so that old browsers can safely ignore new HTML5 constructs. In contrast to HTML 4.01, the HTML5 specification gives detailed rules for lexing and parsing, with the intent that different compliant browsers will produce the same result in the case of incorrect syntax. Although HTML5 now defines a consistent behavior for "tag soup" documents, those documents are not regarded as conforming to the HTML5 standard.