HTML

HTML, which stands for HyperText Markup Language, is the predominant markup language for web pages. It provides a means to create structured documents by denoting structural semantics for text such as headings, paragraphs, lists, links, quotes and other items. It allows images and objects to be embedded and can be used to create interactive forms. It is written in the form of HTML elements consisting of "tags" surrounded by angle brackets within the web page content. It can embed scripts in languages such as JavaScript which affect the behavior of HTML webpages. HTML can also be used to include Cascading Style Sheets (CSS) to define the appearance and layout of text and other material. The W3C, maintainer of both HTML and CSS standards, encourages the use of CSS over explicit presentational markup.

Origins


In 1980, physicist Tim Berners-Lee, who was a contractor at CERN, proposed and prototyped ENQUIRE, a system for CERN researchers to use and share documents. In 1989, Berners-Lee wrote a memo proposing an Internet-based hypertext system. Berners-Lee specified HTML and wrote the browser and server software in the last part of 1990. In that year, Berners-Lee and CERN data systems engineer Robert Cailliau collaborated on a joint request for funding, but the project was not formally adopted by CERN. In his personal notes from 1990 he lists "some of the many areas in which hypertext is used" and puts an encyclopedia first.

First specifications
The first publicly available description of HTML was a document called HTML Tags, first mentioned on the Internet by Berners-Lee in late 1991. It describes 20 elements comprising the initial, relatively simple design of HTML. Except for the hyperlink tag, these were strongly influenced by SGMLguid, an in-house SGML based documentation format at CERN. Thirteen of these elements still exist in HTML 4.

HTML is a text and image formatting language used by web browsers to dynamically format web pages. Many of the text elements are found in the 1988 ISO technical report TR 9537 Techniques for using SGML, which in turn covers the features of early text formatting languages such as that used by the RUNOFF command developed in the early 1960s for the CTSS (Compatible Time-Sharing System) operating system: these formatting commands were derived from the commands used by typesetters to manually format documents. However, the SGML concept of generalized markup is based on elements (nested annotated ranges with attributes) rather than merely print effects, with also the separation of structure and processing; HTML has been progressively moved in this direction with CSS.

Berners-Lee considered HTML to be an application of SGML. It was formally defined as such by the Internet Engineering Task Force (IETF) with the mid-1993 publication of the first proposal for an HTML specification: "Hypertext Markup Language (HTML)" Internet-Draft by Berners-Lee and Dan Connolly, which included an SGML Document Type Definition to define the grammar. The draft expired after six months, but was notable for its acknowledgment of the NCSA Mosaic browser's custom tag for embedding in-line images, reflecting the IETF's philosophy of basing standards on successful prototypes. Similarly, Dave Raggett's competing Internet-Draft, "HTML+ (Hypertext Markup Format)", from late 1993, suggested standardizing already-implemented features like tables and fill-out forms.

After the HTML and HTML+ drafts expired in early 1994, the IETF created an HTML Working Group, which in 1995 completed "HTML 2.0", the first HTML specification intended to be treated as a standard against which future implementations should be based. Published as Request for Comments 1866, HTML 2.0 included ideas from the HTML and HTML+ drafts. The 2.0 designation was intended to distinguish the new edition from previous drafts.

Further development under the auspices of the IETF was stalled by competing interests. Since 1996, the HTML specifications have been maintained, with input from commercial software vendors, by the World Wide Web Consortium (W3C). However, in 2000, HTML also became an international standard (ISO/IEC 15445:2000). The last HTML specification published by the W3C is the HTML 4.01 Recommendation, published in late 1999. Its issues and errors were last acknowledged by errata published in 2001.

HTML version timeline

 * November 24, 1995: HTML 2.0 was published as IETF RFC 1866. Supplemental RFCs added capabilities:
 * November 25, 1995: RFC 1867 (form-based file upload)
 * May 1996: RFC 1942 (tables)
 * August 1996: RFC 1980 (client-side image maps)
 * January 1997: RFC 2070 (internationalization)
 * In June 2000, all of these were declared obsolete/historic by RFC 2854.


 * January 1997: HTML 3.2 was published as a W3C Recommendation. It was the first version developed and standardized exclusively by the W3C, as the IETF had closed its HTML Working Group in September 1996.
 * HTML 3.2 dropped math formulas entirely, reconciled overlap among various proprietary extensions and adopted most of Netscape's visual markup tags. Netscape's blink element and Microsoft's marquee element were omitted due to a mutual agreement between the two companies. A markup for mathematical formulas similar to that in HTML wasn't standardized until 14 months later in MathML.


 * December 1997: HTML 4.0 was published as a W3C Recommendation. It offers three variations:
 * Strict, in which deprecated elements are forbidden,
 * Transitional, in which deprecated elements are allowed,
 * Frameset, in which mostly only frame related elements are allowed;
 * Initially code-named "Cougar", HTML 4.0 adopted many browser-specific element types and attributes, but at the same time sought to phase out Netscape's visual markup features by marking them as deprecated in favor of style sheets. HTML 4 is an SGML application conforming to ISO 8879 - SGML.
 * April 1998: HTML 4.0 was reissued with minor edits without incrementing the version number.
 * December 1999: HTML 4.01 was published as a W3C Recommendation. It offers the same three variations as HTML 4.0 and its last errata were published May 12, 2001.
 * May 2000: ISO/IEC 15445:2000 ("ISO HTML", based on HTML 4.01 Strict) was published as an ISO/IEC international standard. In the ISO this standard falls in the domain of the ISO/IEC JTC1/SC34 (ISO/IEC Joint Technical Committee 1, Subcommittee 34 - Document description and processing languages).
 * As of mid-2008, HTML 4.01 and ISO/IEC 15445:2000 are the most recent versions of HTML. Development of the parallel, XML-based language XHTML occupied the W3C's HTML Working Group through the early and mid-2000s.

HTML draft version timeline

 * October 1991: HTML Tags, an informal CERN document listing twelve HTML tags, was first mentioned in public.
 * June 1992: First informal draft of the HTML DTD, with seven subsequent revisions
 * November 1992: HTML DTD 1.1 (the first with a version number, based on RCS revisions, which start with 1.1 rather than 1.0), an informal draft
 * June 1993: Hypertext Markup Language was published by the IETF IIIR Working Group as an Internet-Draft (a rough proposal for a standard). It was replaced by a second version one month later, followed by six further drafts published by IETF itself  that finally led to HTML 2.0 in RFC1866
 * November 1993: HTML+ was published by the IETF as an Internet-Draft and was a competing proposal to the Hypertext Markup Language draft. It expired in May 1994.
 * April 1995 (authored March 1995): HTML 3.0 was proposed as a standard to the IETF, but the proposal expired five months later without further action. It included many of the capabilities that were in Raggett's HTML+ proposal, such as support for tables, text flow around figures and the display of complex mathematical formulas.
 * W3C began development of its own Arena browser for testing support for HTML 3 and Cascading Style Sheets, but HTML 3.0 did not succeed for several reasons. The draft was considered very large at 150 pages and the pace of browser development, as well as the number of interested parties, had outstripped the resources of the IETF. Browser vendors, including Microsoft and Netscape at the time, chose to implement different subsets of HTML 3's draft features as well as to introduce their own extensions to it. (See Browser wars) These included extensions to control stylistic aspects of documents, contrary to the "belief [of the academic engineering community] that such things as text color, background texture, font size and font face were definitely outside the scope of a language when their only intent was to specify how a document would be organized." Dave Raggett, who has been a W3C Fellow for many years has commented for example, "To a certain extent, Microsoft built its business on the Web by extending HTML features."


 * January 2008: HTML 5 was published as a Working Draft (link) by the W3C.
 * Although its syntax closely resembles that of SGML, HTML 5 has abandoned any attempt to be an SGML application and has explicitly defined its own "html" serialization, in addition to an alternative XML-based XHTML 5 serialization.

XHTML versions
XHTML is a separate language that began as a reformulation of HTML 4.01 using XML 1.0. It continues to be developed:
 * XHTML 1.0, published January 26, 2000 as a W3C Recommendation, later revised and republished August 1, 2002. It offers the same three variations as HTML 4.0 and 4.01, reformulated in XML, with minor restrictions.
 * XHTML 1.1, published May 31, 2001 as a W3C Recommendation. It is based on XHTML 1.0 Strict, but includes minor changes, can be customized, is reformulated using modules from Modularization of XHTML, which was published April 10, 2001 as a W3C Recommendation.
 * XHTML 2.0, . There is no XHTML 2.0 standard. XHTML 2.0 is incompatible with XHTML 1.x and, therefore, would be more accurate to characterize as an XHTML-inspired new language than an update to XHTML 1.x.
 * XHTML 5, which is an update to XHTML 1.x, is being defined alongside HTML 5 in the HTML 5 draft.

Markup
HTML markup consists of several key components, including elements (and their attributes), character-based data types, character references and entity references. Another important component is the document type declaration, which specifies the Document Type Definition. As of HTML 5, no Document Type Definition will need to be specified and will only determine the layout mode.

The Hello world program, a common computer program employed for comparing programming languages, scripting languages and markup languages is made of 9 lines of code in HTML, albeit Newlines are optional:

This Document Type Declaration is for HTML 5.

If the  declaration is not included, Windows Internet Explorer will render using "quirks mode".

Elements
HTML documents are composed entirely of HTML elements that, in their most general form have three components: a pair of element tags with a "start tag" and "end tag"; some element attributes given to the element within the tags; and finally, all the actual textual and graphical information content that will be rendered on the display. An HTML element is everything between and including the tags. A tag is a keyword enclosed in angle brackets.

A common form of an HTML element is: The name of the HTML element is also the name of the tag. Note that the end tag's name starts with a slash character, "/".

The most general form of an HTML element is: By not assigning attributes most start tags default their attribute values.

There are some basic types of tags: Heading of the HTML: ... . Usually the title should be included in the head, for example: Headings:

Paragraph Partition: Newline:. The difference between  and   is that 'br' breaks a line without altering the semantic structure of the page, whereas 'p' sections the page into paragraphs. Here is an example: Annotation: establishes "Golf" as a second-level heading, which would be rendered in a browser in a manner similar to the "HTML markup" title at the start of this section. Structural markup does not denote any specific rendering, but most Web browsers have standardized default styles for element formatting. Text may be further styled with Cascading Style Sheets (CSS).
 * Presentational markup describes the appearance of the text, regardless of its function. For example  indicates that visual output devices should render "boldface" in bold text, but gives no indication what devices which are unable to do this (such as aural devices that read the text aloud) should do. In the case of both   and , there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely   and   respectively. It is easier to see how an aural user agent should interpret the latter two elements. However, they are not equivalent to their presentational counterparts: it would be undesirable for a screen-reader to emphasize the name of a book, for instance, but on a screen such a name would be italicized. Most presentational markup elements have become deprecated under the HTML 4.0 specification, in favor of CSS based style design.
 * Hypertext markup makes parts of a document into links to other documents. HTML up through version XHTML 1.1 requires the use of an anchor element to create a hyperlink in the flow of text: . In addition, the   attribute must be set to a valid URL. For example, the HTML markup, , will render the word " Wikipedia " as a hyperlink. An example to render an image as a hyperlink is:.

Attributes
Most of the attributes of an element are name-value pairs, separated by "=" and written within the start tag of an element after the element's name. The value may be enclosed in single or double quotes, although values consisting of certain characters can be left unquoted in HTML (but not XHTML). Leaving attribute values unquoted is considered unsafe. In contrast with name-value pair attributes, there are some attributes that affect the element simply by their presence in the start tag of the element (like the  attribute for the   element ).

Most elements can take any of several common attributes:


 * The  attribute provides a document-wide unique identifier for an element. This can be used by stylesheets to provide presentational properties: by browsers that focus attention on the specific element, or by scripts to alter the contents or presentation of an element. Appended to the URL of the page, it provides a globally unique identifier for an element, typically a sub-section of the page. For example, the ID "Attributes" in
 * The  attribute provides a way of classifying similar elements. This can be used for semantic or presentation purposes. For example, an HTML document might semantically use the designation   to indicate that all elements with this class value are subordinate to the main text of the document. Presentationally, such elements might be gathered together and presented as footnotes on a page instead of appearing in the place where they occur in the HTML source. Another semantic use of class attributes is in microformats.
 * An author may use the  attribute to assign presentational properties to a particular element. It is considered better practice to use an element’s   or   attributes to select the element from within a stylesheet, though sometimes this can be too cumbersome for a simple and specific or ad hoc application of styled properties.
 * The  attribute is used to attach subtextual explanation to an element. In most browsers this attribute is displayed as what is often referred to as a tooltip.
 * The  attribute identifies the natural language of the element's contents, if different from that of the rest of the document. For example, in an English-language document:

The abbreviation element,, can be used to demonstrate some of these attributes:



This example displays as HTML ; in most browsers, pointing the cursor at the abbreviation should display the title text "Hypertext Markup Language."

Most elements also take the language-related attribute.

Character and entity references
As of version 4.0, HTML defines a set of 252 character entity references and a set of 1,114,050 numeric character references, both of which allow individual characters to be written via simple markup, rather than literally. A literal character and its markup counterpart are considered equivalent and are rendered identically.

The ability to "escape" characters in this way allows for the characters  and   (when written as   and , respectively) to be interpreted as character data, rather than markup. For example, a literal  normally indicates the start of a tag, and   normally indicates the start of a character entity reference or numeric character reference; writing it as   or   or   allows   to be included in the content of elements or the values of attributes. The double-quote character, when used to quote an attribute value, must also be escaped as  or   or   when it appears within the attribute value itself. The single-quote character, when used to quote an attribute value, must also be escaped as  or   (should NOT be escaped as   except in XHTML documents ) when it appears within the attribute value itself. However, since document authors often overlook the need to escape these characters, browsers tend to be very forgiving, treating them as markup only when subsequent text appears to confirm that intent.

Escaping also allows for characters that are not easily typed or that aren't even available in the document's character encoding to be represented within the element and attribute content. For example, the acute-accented , a character typically found only on Western European keyboards, can be written in any HTML document as the entity reference   or as the numeric references   or. The characters comprising those references (that is, the, the  , the letters in  , and so on) are available on all keyboards and are supported in all character encodings, whereas the literal   is not.

Data types
HTML defines several data types for element content, such as script data and stylesheet data, and a plethora of types for attribute values, including IDs, names, URIs, numbers, units of length, languages, media descriptors, colors, character encodings, dates and times, and so on. All of these data types are specializations of character data.

Document type declaration
HTML documents are required to start with a Document Type Declaration (informally, a "doctype"). In browsers, the function of the doctype is to indicate the rendering mode—particularly to avoid quirks mode.

The original purpose of the doctype was to enable parsing and validation of HTML documents by SGML tools based on the Document Type Definition (DTD). The DTD to which the DOCTYPE refers contains machine-readable grammar specifying the permitted and prohibited content for a document conforming to such a DTD. Browsers, on the other hand, do not implement HTML as an application of SGML and by consequence do not read the DTD. HTML 5 does not define a DTD, because of the technology's inherent limitations, so in HTML 5 the doctype declaration,, does not refer to a DTD.

An example of an HTML 4 doctype is

This declaration references the DTD for the Strict version of HTML 4.01, which does not include presentational elements like, leaving formatting to Cascading Style Sheets and the   and   element. SGML-based validators read the DTD in order to properly parse the document and to perform validation. In modern browsers, this doctype activates standards mode as opposed to quirks mode.

In addition, HTML 4.01 provides Transitional and Frameset DTDs, as explained below.

Semantic HTML
Semantic HTML is a way of writing HTML that emphasizes the meaning of the encoded information over its presentation (look). HTML has included semantic markup from its inception, but has also included presentational markup such as. The introduction of this shorthand, which is not used in the SGML declaration for HTML 4.01, may confuse earlier software unfamiliar with this new convention. A fix for this is to include a space before closing the tag, as such:.

To understand the subtle differences between HTML and XHTML, consider the transformation of a valid and well-formed XHTML 1.0 document that adheres to Appendix C (see below) into a valid HTML 4.01 document. To make this translation requires the following steps:
 * 1) The language for an element should be specified with a   attribute rather than the XHTML   attribute. XHTML uses XML's built in language-defining functionality attribute.
 * 2) Remove the XML namespace . HTML has no facilities for namespaces.
 * 3) Change the document type declaration from XHTML 1.0 to HTML 4.01. (see DTD section for further explanation).
 * 4) If present, remove the XML declaration. (Typically this is:  ).
 * 5) Ensure that the document’s MIME type is set to  . For both HTML and XHTML, this comes from the HTTP   header sent by the server.
 * 6) Change the XML empty-element syntax to an HTML style empty element (  to  ).

Those are the main changes necessary to translate a document from XHTML 1.0 to HTML 4.01. To translate from HTML to XHTML would also require the addition of any omitted opening or closing tags. Whether coding in HTML or XHTML it may just be best to always include the optional tags within an HTML document rather than remembering which tags can be omitted.

A well-formed XHTML document adheres to all the syntax requirements of XML. A valid document adheres to the content specification for XHTML, which describes the document structure.

The W3C recommends several conventions to ensure an easy migration between HTML and XHTML (see HTML Compatibility Guidelines). The following steps can be applied to XHTML 1.0 documents only:
 * Include both  and   attributes on any elements assigning language.
 * Use the empty-element syntax only for elements specified as empty in HTML.
 * Include an extra space in empty-element tags: for example  instead of.
 * Include explicit close tags for elements that permit content but are left empty (for example, , not  ).
 * Omit the XML declaration.

By carefully following the W3C’s compatibility guidelines, a user agent should be able to interpret the document equally as HTML or XHTML. For documents that are XHTML 1.0 and have been made compatible in this way, the W3C permits them to be served either as HTML (with a  MIME type), or as XHTML (with an   or   MIME type). When delivered as XHTML, browsers should use an XML parser, which adheres strictly to the XML specifications for parsing the document's contents.

Transitional versus strict
HTML 4 defined three different versions of the language: Strict, Transitional (once called Loose) and Frameset. The Strict version is intended for new documents and is considered best practice, while the Transitional and Frameset versions were developed to make it easier to transition documents that conformed to older HTML specification or didn't conform to any specification to a version of HTML 4. The Transitional and Frameset versions allow for presentational markup, which is omitted in the Strict version. Instead, cascading style sheets are encouraged to improve the presentation of HTML documents.

Because XHTML 1 only defines an XML syntax for the language defined by HTML 4, the same differences apply to XHTML 1 as well.

The Transitional version allows the following parts of the vocabulary, which are not included in the Strict version:
 * A looser content model
 * Inline elements and plain text are allowed directly in:,  ,  ,   and
 * Presentation related elements
 * underline
 * strike-through
 * Presentation related attributes
 * and  attributes for   element.
 * attribute on,  , paragraph  and heading ( ... ) elements
 * ,,   and   attributes on   element
 * ,,   and   attributes on   and   elements
 * attribute on  and   elements
 * and  on   element
 * ,,  ,   on   and   elements
 * attribute on  element
 * attribute on  element
 * attribute on,   and   elements
 * ,  and   attributes on   and   elements
 * and  attributes on   element
 * attribute on  element
 * Additional elements in Transitional specification
 * list (no substitute, though unordered list is recommended)
 * list (no substitute, though unordered list is recommended)
 * (element requires server-side support and is typically added to documents server-side,  and   elements can be used as a substitute)
 * (deprecated in favor of object element)
 * The  attribute on script element (redundant with the   attribute).
 * Frame related entities
 * attribute on, client-side image-map ,  ,   and   elements
 * The  attribute on script element (redundant with the   attribute).
 * Frame related entities
 * attribute on, client-side image-map ,  ,   and   elements
 * attribute on, client-side image-map ,  ,   and   elements
 * attribute on, client-side image-map ,  ,   and   elements

The Frameset version includes everything in the Transitional version, as well as the  element (used instead of  ) and the   element.

Frameset versus transitional
In addition to the above transitional differences, the frameset specifications (whether XHTML 1.0 or HTML 4.01) specifies a different content model, with  replacing , that contains either   elements, or optionally   with a.

Summary of specification versions
As this list demonstrates, the loose versions of the specification are maintained for legacy support. However, contrary to popular misconceptions, the move to XHTML does not imply a removal of this legacy support. Rather the X in XML stands for extensible and the W3C is modularizing the entire specification and opening it up to independent extensions. The primary achievement in the move from XHTML 1.0 to XHTML 1.1 is the modularization of the entire specification. The strict version of HTML is deployed in XHTML 1.1 through a set of modular extensions to the base XHTML 1.1 specification. Likewise, someone looking for the loose (transitional) or frameset specifications will find similar extended XHTML 1.1 support (much of it is contained in the legacy or frame modules). The modularization also allows for separate features to develop on their own timetable. So for example, XHTML 1.1 will allow quicker migration to emerging XML standards such as MathML (a presentational and semantic math language based on XML) and XForms&mdash;a new highly advanced web-form technology to replace the existing HTML forms.

In summary, the HTML 4.01 specification primarily reined in all the various HTML implementations into a single clearly written specification based on SGML. XHTML 1.0, ported this specification, as is, to the new XML defined specification. Next, XHTML 1.1 takes advantage of the extensible nature of XML and modularizes the whole specification. XHTML 2.0 will be the first step in adding new features to the specification in a standards-body-based approach.

Hypertext features not in HTML
HTML lacks some of the features found in earlier hypertext systems, such as typed links, source tracking, fat links and others. Even some hypertext features that were in early versions of HTML have been ignored by most popular web browsers until recently, such as the link element and in-browser Web page editing.

Sometimes Web services or browser manufacturers remedy these shortcomings. For instance, wikis and content management systems allow surfers to edit the Web pages they visit.

WYSIWYG Editors
There are some WYSIWYG editors in which the user lays out everything as it is to appear in the HTML document using a graphical user interface, where the editor renders this as an HTML document, no longer requiring the author to have extensive knowledge of HTML.

Web page editing is clearly dominated by the WYSIWYG editing model. But, this model has been criticized, primarily because of the low quality of the generated code; there are voices advocating a change to the WYSIWYM model.

WYSIWYG editors remains a controversial topic because of their perceived flaws such as:


 * Relying mainly on layout as opposed to meaning, often using markup that does not convey the intended meaning but simply copies the layout.
 * Often producing extremely verbose and redundant code that fails to make use of the cascading nature of HTML and CSS.
 * Often producing ungrammatical markup often called tag soup.
 * As a great deal of information of HTML documents is not in the layout, the model has been criticized for its 'what you see is all you get'-nature.

Nevertheless, since WYSIWYG editors offer convenience over hand-coded pages as well as not requiring the author to know the finer details of HTML, they still dominate web authoring.

HTML tutorials

 * HTML Dog
 * HTML Tutorials
 * HTML.net
 * Your HTML Source
 * HTML tutorial
 * The How To Guide To Learning HTML

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