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HTML5 is the proposed next standard for HTML 4.01, XHTML 1.0 and DOM Level 2 HTML. It aims to reduce the need for proprietary plug-in-based rich internet application (RIA) technologies such as Adobe Flash, Microsoft Silverlight, and Sun JavaFX.
The ideas behind HTML5 were pioneered in 2004 by the WHATWG; HTML5 incorporates Web Forms 2.0, another WHATWG specification. The HTML5 specification was adopted as the starting point of the work of the new HTML working group of the W3C in 2007. This working group published the First Public Working Draft of the specification on January 22, 2008. The specification is an ongoing work, and is expected to remain so for many years, although parts of HTML5 are going to be finished and implemented in browsers before the whole specification reaches final Recommendation status.
HTML5 introduces a number of new elements and attributes that reflect typical usage on modern Web sites. Some of them are semantic replacements for common uses of generic block (Template:Tag) and inline (Template:Tag) elements, for example Template:Tag (website navigation block) and Template:Tag. Other elements provide new functionality through a standardized interface, such as the Template:Tag and Template:Tag elements.
Some deprecated elements from HTML 4.01 have been dropped, including purely presentational elements such as Template:Tag and Template:Tag, whose effects are achieved using CSS. There is also a renewed emphasis on the importance of DOM scripting in Web behavior.
The HTML5 syntax is no longer based on SGML despite the similarity of its markup. It has, however, been designed to be backward compatible with common parsing of older versions of HTML. It comes with a new introductory line that looks like an SGML document type declaration,
<!DOCTYPE html>, which enables standards-compliant rendering in all browsers that use "DOCTYPE sniffing".
In addition to specifying markup, HTML5 specifies scripting application programming interfaces (APIs). Existing document object model (DOM) interfaces are extended and de facto features documented. There are also new APIs, such as:
- The canvas element for immediate mode 2D drawing
- Timed media playback
- Offline storage database
- Document editing
- Cross-document messaging
- Browser history management
- MIME type and protocol handler registration
Some of the new features are part of HTML5 mainly because there are no volunteers to split HTML5 and maintain separate specifications of these features.
Differences from HTML 4.01/XHTML 1.X
The following is a cursory list of differences and some specific examples.
- New parsing rules oriented towards flexible parsing and compatibility; not based on SGML
- Ability to use inline SVG and MathML in
- New elements –
- New types of form controls – dates and times,
- New attributes –
- Global attributes (that can be applied for every element) –
data-*(custom data attributes)
- Forms will get support for PUT and DELETE methods too instead of just GET and POST (see Representational State Transfer for use cases)
- Deprecated elements dropped –
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.
Ian Hickson, editor of the HTML5 specification, expects the specification to reach the W3C Candidate Recommendation stage during 2012, and W3C Recommendation in the year 2022 or later. However, many parts of the specification are stable and may be implemented in products:
Some sections are already relatively stable and there are implementations that are already quite close to completion, and those features can be used today (e.g. <canvas>).
– WHAT Working Group, When will HTML5 be finished?, FAQ
According to the W3C timetable, it is estimated that HTML5 will reach W3C Recommendation by late 2010. However, the First Public Working Draft estimate was missed with 8 months, and Last Call and Candidate Recommendation were expected to be reached in 2008 but as of late 2009 HTML5 had not reached W3C Last Call. 
- ↑ 1.0 1.1 "HTML 5: A vocabulary and associated APIs for HTML and XHTML.". W3C. http://www.w3.org/TR/html5/. Retrieved 2009-01-28.
- ↑ "When will HTML5 be finished?". WHATWG. WHATWG Wiki. http://wiki.whatwg.org/wiki/FAQ#When_will_HTML5_be_finished.3F. Retrieved 2009-09-10.
- ↑ IBM Developer Works New elements in HTML5: Structure and semantics
- ↑ HTML5 differences from HTML4 – APIs W3C
- ↑ Hickson, Ian (2008-10-27). "HTML5 Specification – List of sections and corresponding work estimates". firstname.lastname@example.org mailing list. http://lists.w3.org/Archives/Public/public-html/2008Oct/0127.html. Retrieved 2008-12-10.
- ↑ "FAQ – WHATWG Wiki". WHATWG. http://wiki.whatwg.org/wiki/FAQ. Retrieved 2008-02-25.
- ↑ 7.0 7.1 "When will HTML5 be finished?". FAQ. WHAT Working Group. http://wiki.whatwg.org/wiki/FAQ#When_will_HTML5_be_finished.3F. Retrieved 2009-11-29.
- ↑ http://www.w3.org/2007/03/HTML-WG-charter.html#deliverables
- WHAT Working Group
- W3C HTML Working Group
- WHATWG Editor's draft
- W3C Editor's draft
- HTML5 differences from HTML4
- HTML 5 Reference
- HTML Design Principles
- HTML 5: The Markup Language
- HTML 5 Quick Reference Guide
- Compatibility tables for features in HTML5, CSS3, SVG and other upcoming web technologies