What is HTML5

HTML5 has not yet appeared in developing HTML markup language (currently: 4.01).

An initial proposal for HTML5 was first released in mid-2004 under the name of WHATWG Web Applications 1.0.

Near the end of 2006, Tim Berners-Lee announced that a new working group would arise to develop HTML. This had the W3C creating competition in their own home because they also worked on the HTML language similar to XHTML 2.0, a purely XML-based format.  In July 2009, however, it was announced that its development was set for late 2009.

Between November 2006 and March 2007, the existing working groups at the W3C were redesigned. The new HTML Working Group was designed to develop a language that is geared to the classic HTML syntax, but also one in which an XML formulation can be used. In addition, both languages should allow a very similar memory mapping of the language (i.e., a representation by the DOM).
In a vote in May 2007, the members of the HTML Working Group decided that the specification of the WHATWG Web Applications 1.0 was to be used as a starting point for discussion and further development of HTML.
On January 22, 2008, W3C published the first working draft of HTML5. On June 10, 2008, this was followed by the second working draft, which integrated the Web Forms 2.0 specification for the first time.  On February 12, 2009, the third draft followed; on April 23, the fourth, on August 25, the fifth; and on December 7, the sixth.
The specification itself notes that some sections are more mature than others. Some of the more mature developments are already reflected in current browser versions and may not be explicitly used.
Goals

HTML5 will replace the existing specifications (HTML 4.01, XHTML 1.0 and DOM 2 HTML). The draft definition, therefore, has three linguistic forms: HTML5, XHTML5 and DOM 5 HTML.
With the emergence of the HTML Working Group, the aims and specifications the language will contain have been established.

The following is a short and incomplete overview:
The language will have evolved from HTML 4.01.
This language will have classic HTML, XML as a formulation, and DOM mapping can be used.
For the version that is classic HTML-based, an error checking tool is available.
The DOM representation of the dynamic programming language must possess work with this language.
There will be interfaces for working with linked multimedia content.

As a first, public, working draft of the HTML Working Group, the “HTML Design Principles” was published. This contained further objectives and policies to be decided upon.  These included such issues as whether something old can be improved, and whether something needs to be redeveloped. In the latter case, there are guidelines that may lead to new development (e.g., alternative content for multimedia objects are available).