HTML
{{Infobox file format
| name = HTML (HyperText Markup Language)<!-- For rollover expansion of links in other pages-->
| icon =
| screenshot = [[Image:HTML.svg|150px]]
| extension = .html, .htm
| mime = text/html
| type code = TEXT
| uniform type = public.html
| owner = [[World Wide Web Consortium]]
| genre = [[Markup language]]
| container for = 
| contained by =
| extended from = [[Standard Generalized Markup Language|SGML]]
| extended to = [[XHTML]]
| standard = [http://www.w3.org/TR/1999/REC-html401-19991224/ W3C HTML 4.01]<br />
[http://www.w3.org/TR/REC-html32-19970114 W3C HTML 3.2]
}}
'''HTML''', an [[Acronym and initialism|initialism]] of '''HyperText Markup Language''', is the predominant [[markup language]] for [[web page]]s. It provides a means to describe the structure of text-based information in a document — by denoting certain text as links, headings, paragraphs, lists, and so on — and to supplement that text with ''interactive forms'', embedded ''images'', and other objects. HTML is written in the form of tags, surrounded by [[Brackets#Angle brackets or chevrons .3C .3E|angle brackets]]. HTML can also describe, to some degree, the appearance and [[semantics]] of a document, and can include embedded [[scripting language]] code (such as JavaScript) which can affect the behavior of [[Web browser]]s and other HTML processors.

HTML is also often used to refer to content in specific languages, such as a [[MIME type]] text/html, or even more broadly as a generic term for HTML, whether in its
[[XML]]-descended form (such as [[XHTML]] 1.0 and later) or its form descended directly from [[SGML]] (such as HTML 4.01 and earlier).

By convention, HTML format data files use a file extension .html or .htm.

==History of HTML==
===Origins===
In 1980, physicist [[Tim Berners-Lee]], who was an independent contractor at [[CERN]], proposed and prototyped [[ENQUIRE]], a  system for CERN researchers to use and share documents. In 1989, Berners-Lee and CERN data systems engineer [[Robert Cailliau]] each submitted separate proposals for an [[Internet]]-based [[hypertext]] system providing similar functionality. The following year, they collaborated on a joint proposal, the WorldWideWeb (W3) project,<ref>Tim Berners-Lee, "Information Management: A Proposal." CERN (March 1989, May 1990).  http://www.w3.org/History/1989/proposal.html</ref>
which was accepted by CERN.

===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.<ref>{{cite web|url=http://www.w3.org/History/19921103-hypertext/hypertext/WWW/MarkUp/Tags.html|title=HTML Tags|publisher=World Wide Web Consortium|accessdate=2007-04-08}}</ref><ref>{{cite web|url=http://lists.w3.org/Archives/Public/www-talk/1991SepOct/0003.html|title=First mention of HTML Tags on the www-talk mailing list|publisher=World Wide Web Consortium|date=[[1991-10-29]]|accessdate=2007-04-08}}</ref> It describes 22 elements comprising the initial, relatively simple design of HTML. Thirteen of these elements still exist in HTML 4.<ref>{{cite web|url=http://www.w3.org/TR/1999/REC-html401-19991224/index/elements|title=Index of elements in HTML 4|publisher=World Wide Web Consortium|date=[[1999-12-24]]|accessdate=2007-04-08}}</ref>

Berners-Lee considered HTML to be, at the time, an application of [[SGML]], but it was not formally defined as such until the mid-1993 publication, by the [[Internet Engineering Task Force|IETF]], of the first proposal for an HTML specification: Berners-Lee and [[Dan Connolly]]'s "Hypertext Markup Language (HTML)" Internet-Draft, which included an SGML [[Document Type Definition]] to define the grammar.<ref>{{cite web|url=http://lists.w3.org/Archives/Public/www-talk/1991NovDec/0020.html|title=Re: SGML/HTML docs, X Browser (archived www-talk mailing list post)|author=Tim Berners-Lee|date=[[1991-12-09]]|accessdate=2007-06-16|quote=SGML is very general. HTML is a specific application of the SGML basic syntax applied to hypertext documents with simple structure.}}</ref> The draft expired after six months, but was notable for its acknowledgment of the [[Mosaic (web browser)|NCSA Mosaic]] browser's custom tag for embedding in-line images, reflecting the IETF's philosophy of basing standards on successful prototypes.<ref name="raymond">{{cite book|url=http://www.faqs.org/docs/artu/|chapterurl=http://www.faqs.org/docs/artu/ietf_process.html|title=[[The Art of Unix Programming]]|last=Raymond|first=Eric|chapter=IETF and the RFC Standards Process|quote=In IETF tradition, standards have to arise from experience with a working prototype implementation — but once they become standards, code that does not conform to them is considered broken and mercilessly scrapped. …Internet-Drafts are not specifications, and software implementers and vendors are specifically barred from claiming compliance with them as if they were specifications. Internet-Drafts are focal points for discussion, usually in a working group… Once an Internet-Draft has been published with an RFC number, it is a specification to which implementers may claim conformance. It is expected that the authors of the RFC and the community at large will begin correcting the specification with field experience.}}</ref> 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.<ref name="html+">{{cite web|url=https://datatracker.ietf.org/public/idindex.cgi?command=id_detail&id=789|title=HTML+ Internet-Draft - Abstract|quote=Browser writers are experimenting with extensions to HTML and it is now appropriate to draw these ideas together into a revised document format. The new format is designed to allow a gradual roll over from HTML, adding features like tables, captioned figures and fill-out forms for querying remote databases or mailing questionnaires.}}</ref>

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.<ref name="raymond"/> Published as [[Request for Comments]] 1996, HTML 2.0 included ideas from the HTML and HTML+ drafts.<ref>{{cite web|url=http://www.ietf.org/rfc/rfc1866.txt|title=RFC 1866: Hypertext Markup Language - 2.0 - Acknowledgments|publisher=Internet Engineering Task Force|date=[[2005-09-22]]|accessdate=2007-06-16|quote=Since 1993, a wide variety of Internet participants have contributed to the evolution of HTML, which has included the addition of in-line images introduced by the NCSA Mosaic software for WWW. Dave Raggett played an important role in deriving the forms material from the HTML+ specification. Dan Connolly and Karen Olson Muldrow rewrote the HTML Specification in 1994. The document was then edited by the HTML working group as a whole, with updates being made by Eric Schieler, Mike Knezovich, and Eric W. Sink at Spyglass, Inc. Finally, Roy Fielding restructured the entire draft into its current form.}}</ref> There was no "HTML 1.0"; the 2.0 designation was intended to distinguish the new edition from previous drafts.<ref>{{cite web|url=http://www.ietf.org/rfc/rfc1866.txt|title=RFC 1866: Hypertext Markup Language - 2.0 - Introduction|publisher=Internet Engineering Task Force|date=[[2005-09-22]]|accessdate=2007-06-16|quote=This document thus defines an HTML 2.0 (to distinguish it from the previous informal specifications). Future (generally upwardly compatible) versions of HTML with new features will be released with higher version numbers.}}</ref>

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).<ref name="raggett"/> However, in 2000, HTML also became an international standard ([[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|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.

===Version history of the standard===
{{Html series}}
====HTML versions====
'''July, 1993:''' [http://www.w3.org/MarkUp/draft-ietf-iiir-html-01.txt Hypertext Markup Language], was published at [[Internet Engineering Task Force|IETF]] working draft (that is, not yet a standard).

'''November, 1995:''' [http://tools.ietf.org/html/rfc1866 HTML 2.0] published as IETF [[Request for Comments]]:
* RFC 1866,
* supplemented by RFC 1867 (form-based file upload) that same month,
* RFC 1942 (tables) in ''May 1996'',
* RFC 1980 (client-side image maps) in ''August 1996'', and
* RFC 2070 ([[internationalization and localization|internationalization]]) in ''January 1997'';

Ultimately, all were declared obsolete/historic by RFC 2854 in ''June 2000''.

'''April 1995''': [http://www.w3.org/MarkUp/html3/ HTML 3.0], proposed as a standard to the IETF.<!--authored March 1995--> 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.<ref>{{cite web
|url=http://www.w3.org/MarkUp/html3/CoverPage
|title=HyperText Markup Language Specification Version 3.0
|accessdate=2007-06-16}}</ref>
A demonstration appeared in W3C's own [[Arena (web browser)|Arena browser]]. HTML 3.0 did not succeed for several reasons. The pace of browser development, as well as the number of interested parties, had outstripped the resources of the IETF.<ref name="raggett">{{cite book|first=Dave|last=Raggett|title=Raggett on HTML 4|year=1998|url=http://www.w3.org/People/Raggett/book4/ch02.html|accessdate=2007-07-09}}</ref>
Netscape continued to introduce HTML elements that specified the visual appearance of documents,<ref>{{cite web
|url=http://wp.netscape.com/assist/net_sites/html_extensions_3.html
|title=Extensions to HTML 3.0
|publisher=Netscape
|quote=Netscape remains committed to supporting HTML 3.0. To that end, we've gone ahead and implemented several of the more stable proposals, in expectation that they will be approved. …In addition, we've also added several new areas of HTML functionality to Netscape Navigator that are not currently in the HTML 3.0 specification. We think they belong there, and as part of the standards process, we are proposing them for inclusion.}}</ref>
contrary to the goals of the newly-formed W3C, which sought to limit HTML to describing logical structure.<ref>http://www.w3.org/Press/CSS2</ref>
Microsoft, a newcomer at the time, played to all sides by creating its own tags, implementing Netscape's elements for compatibility, and supporting W3C features such as Cascading Style Sheets.<ref name="raggett" />

'''[[January 14]], [[1997]]:''' [http://www.w3.org/TR/REC-html32 HTML 3.2], 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 1997.<ref>{{cite web
|url=http://www.w3.org/MarkUp/HTML-WG/
|title=IETF HTML WG
|accessdate=2007-06-16
|quote=NOTE: This working group is closed}}</ref>
The new version 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.<ref name="raggett"/> The ability to include mathematical formulas in HTML would not be standardized until years later in [[MathML]].

'''[[December 18]], [[1997]]:''' [http://www.w3.org/TR/REC-html40-971218/ HTML 4.0], published as a W3C Recommendation. It offers three "flavors":
* Strict, in which deprecated elements are forbidden,
* Transitional, in which deprecated elements are allowed,
* Frameset, in which mostly only [[Framing (World Wide Web)|frame]] related elements are allowed;

HTML 4.0 (initially code-named "Cougar")<ref name="engelfriet">{{cite web
|url=http://htmlhelp.com/reference/wilbur/intro.html
|title=Introduction to Wilbur
|author=Arnoud Engelfriet
|publisher=Web Design Group
|accessdate=2007-06-16}}</ref>
likewise 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 [[deprecation|deprecated]] in favor of style sheets. Minor editorial revisions to the HTML 4.0 specification were published in 1998 without incrementing the version number and further minor revisions as HTML 4.01.

'''[[April 24]], [[1998]]:''' [http://www.w3.org/TR/1998/REC-html40-19980424/ HTML 4.0] was reissued with minor edits without incrementing the version number.

'''[[December 24]], [[1999]]:''' [http://www.w3.org/TR/html401 HTML 4.01], published as a W3C Recommendation. It offers the same three flavors as HTML 4.0, and its last [http://www.w3.org/MarkUp/html4-updates/errata errata] were published [[May 12]], [[2001]].

HTML 4.01 and ISO/IEC 15445:2000 are the most recent and final versions of HTML.

'''[[May 15]], [[2000]]:''' [https://www.cs.tcd.ie/15445/15445.HTML ISO/IEC 15445:2000] ("[[International Organization for Standardization|ISO]] HTML", based on HTML 4.01 Strict), published as an ISO/IEC international standard.

'''[[January 22]], [[2008]]:''' [http://www.w3.org/TR/html5/ HTML 5], published as a Working Draft by W3C.

====XHTML versions====
{{main|XHTML}}
XHTML is a separate language that began as a reformulation of HTML 4.01 using XML 1.0. It continues to be developed:
* [http://www.w3.org/TR/xhtml1/ XHTML 1.0], published [[January 26]], [[2000]] as a W3C Recommendation, later revised and republished [[August 1]], [[2002]]. It offers the same three flavors as HTML 4.0 and 4.01, reformulated in XML, with minor restrictions.
* [http://www.w3.org/TR/xhtml11/ 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, and is reformulated using modules from [http://www.w3.org/TR/xhtml-modularization Modularization of XHTML], which was published [[April 10]], [[2001]] as a W3C Recommendation.
* [http://www.w3.org/TR/xhtml2/ XHTML 2.0] is still a W3C Working Draft. 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 [http://www.w3.org/html/wg/html5/ HTML 5 draft].

==HTML markup==
HTML markup consists of several key components, including ''elements'' (and their ''attributes''), character-based ''data types'', and ''character references'' and ''entity references''. Another important component is the ''document type declaration''.
HTML [[Hello world program|Hello World]]:
<source lang="html4strict">
<html>
<head>
<title>Hello HTML</title>
</head>
<body>
<span>Hello World!</span>
</body>
</html>
</source>

===Elements===
:''See [[HTML element]]s for more detailed descriptions.''
Elements are the basic structure for HTML markup. Elements have two basic properties: attributes and content. Each attribute and each element's content has certain restrictions that must be followed for an HTML document to be considered valid. An element usually has a start tag (e.g. <code><element-name></code>) and an end tag (e.g. <code></element-name></code>). The element's attributes are contained in the start tag and content is located between the tags (e.g. <code><element-name&nbsp;attribute="value">Content</element-name></code>). Some elements, such as <code><nowiki><br></nowiki></code>, do not have any content and must not have a closing tag. Listed below are several types of markup elements used in HTML.

'''Structural''' markup describes the purpose of text. For example, <code><nowiki><h2>Golf</h2></nowiki></code> 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 on how elements should be formatted. Text may be further styled with [[Cascading Style Sheets]] (CSS).

'''Presentational''' markup describes the appearance of the text, regardless of its function. For example <code><nowiki><b>boldface</b></nowiki></code> 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 <code><nowiki><b>bold</b></nowiki></code> and <code><nowiki><i>italic</i></nowiki></code>, there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely <code><nowiki><strong>strong emphasis</strong></nowiki></code> and <code><nowiki><em>emphasis</em></nowiki></code> 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 [[Deprecation|deprecated]] under the HTML 4.0 specification, in favor of [[Cascading Style Sheets|CSS]] based style design.

'''Hypertext''' markup links parts of the document 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: <code><nowiki><a>Wikipedia</a></nowiki></code>. However, the <code>href</code> attribute must also be set to a valid [[Uniform Resource Locator|URL]] so for example the HTML code, <code><nowiki><a&nbsp;href="http://en.wikipedia.org/">Wikipedia</a></nowiki></code>, will render the word "<span class="plainlinks">[http://en.wikipedia.org/ Wikipedia]</span>" as a [[hyperlink]].To link on an image, the anchor tag use the following syntax: <a href="url"><img src="image.gif" /></a>

===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).<ref>{{cite web|url=http://www.w3.org/TR/html401/intro/sgmltut.html#h-3.2.2|title=www.w3.org/TR/html401/intro/sgmltut.html#h-3.2.2<!--INSERT TITLE-->}}</ref><ref>{{cite web|url=http://www.w3.org/TR/xhtml1/diffs.html#h-4.4|title=www.w3.org/TR/xhtml1/diffs.html#h-4.4<!--INSERT TITLE-->}}</ref> Leaving attribute values unquoted is considered unsafe.<ref>{{cite web|url=http://www.cs.tut.fi/~jkorpela/qattr.html|title=www.cs.tut.fi/~jkorpela/qattr.html<!--INSERT TITLE-->}}</ref> 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<ref>{{cite web|url=http://www.w3.org/History/19921103-hypertext/hypertext/WWW/MarkUp/Tags.html|title=www.w3.org/History/19921103-hypertext/hypertext/WWW/MarkUp/Tags.html<!--INSERT TITLE-->}}</ref> (like the <code>ismap</code> attribute for the <code>img</code> element<ref>{{cite web|url=http://www.w3.org/TR/1999/REC-html401-19991224/struct/objects.html#adef-ismap|title=www.w3.org/TR/1999/REC-html401-19991224/struct/objects.html#adef-ismap<!--INSERT TITLE-->}}</ref>).

Most elements can take any of several common attributes:

* The <code>id</code> attribute provides a document-wide unique identifier for an element. This can be used by stylesheets to provide presentational properties, by browsers to focus attention on the specific element, or by scripts to alter the contents or presentation of an element. 

* The <code>class</code> attribute provides a way of classifying similar elements for presentation purposes. For example, an HTML document might use the designation <code>class="notation"</code> to indicate that all elements with this class value are subordinate to the main text of the document. 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.

* An author may use the <code>style</code> non-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s son- <code>id</code> page and select the element with a stylesheet, though sometimes this can be too cumbersome for a simple ad hoc application of styled properties. 

* The <code>title</code> 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 generic inline element <code>span</code> can be used to demonstrate these various attributes:

::<source lang="html4strict"><span id="anId" class="aClass" style="color:blue;" title="Hypertext Markup Language">HTML</span></source>

This example displays as <span id="anId" class="aClass" style="color:blue;" title="Hypertext Markup Language">HTML</span>; in most browsers, pointing the cursor at the abbreviation should display the title text "Hypertext Markup Language."

Most elements also take the language-related attributes <code>lang</code> and <code>dir</code>.

===Character and entity references===
As of version 4.0, HTML defines a set of [[List of XML and HTML character entity references|252]] [[character entity reference]]s and a set of 1,114,050 [[numeric character reference]]s, 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 <code>&lt;</code> and <code>&amp;</code> (when written as <code>&amp;lt;</code> and <code>&amp;amp;</code>, respectively) to be interpreted as character data, rather than markup. For example, a literal <code>&lt;</code> normally indicates the start of a tag, and <code>&amp;</code> normally indicates the start of a character entity reference or numeric character reference; writing it as <code>&amp;amp;</code> or <code>&amp;#x26;</code> or <code>&amp;#38;</code> allows <code>&amp;</code> to be included in the content of elements or the values of attributes. The double-quote character (<code>"</code>), when used to quote an attribute value, must also be escaped as <code>&amp;quot;</code> or <code>&amp;#x22;</code> or <code>&amp;#34;</code> when it appears within the attribute value itself. The single-quote character (<code>'</code>), when used to quote an attribute value, must also be escaped as <code>&amp;#x27;</code> or <code>&amp;#39;</code> (should NOT be escaped as <code>&amp;apos;</code> 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 <code>e</code> (<code>é</code>), a character typically found only on Western European keyboards, can be written in any HTML document as the entity reference <code>&amp;eacute;</code> or as the numeric references <code>&amp;#233;</code> or <code>&amp;#xE9;</code>. The characters comprising those references (that is, the <code>&amp;</code>, the <code>;</code>, the letters in <code>eacute</code>, and so on) are available on all keyboards and are supported in all character encodings, whereas the literal <code>é</code> is not.

===Data types===
HTML defines several [[data type]]s 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.

===The Document Type Declaration===
In order to enable [[Document Type Definition]] (DTD)-based validation with SGML tools and in order to avoid the [[quirks mode]] in browsers, HTML documents can start with a [[Document Type Declaration]] (informally, a "DOCTYPE"). 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 do not necessarily read the DTD, however. The most popular graphical browsers use DOCTYPE declarations (or the lack thereof) and other data at the beginning of sources to determine which rendering mode to use.

For example:

:<code><nowiki><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"></nowiki></code>

This declaration references the Strict DTD of HTML 4.01, which does not have presentational elements like <code><nowiki><font></nowiki></code>, leaving formatting to Cascading Style Sheets and the <code>span</code> and <code>div</code> tags. SGML-based validators read the DTD in order to properly parse the document and to perform validation. In modern browsers, the HTML 4.01 Strict doctype activates standards layout mode for [[Cascading Style Sheets|CSS]] as opposed to [[quirks mode]].

In addition, HTML 4.01 provides Transitional and Frameset DTDs. The Transitional DTD was intended to gradually phase in the changes made in the Strict DTD, while the Frameset DTD was intended for those documents which contained frames.

==Semantic HTML==
There is no official specification called "Semantic HTML", though the strict flavors of HTML discussed [[#Current flavors of HTML|below]] are a push in that direction. Rather, semantic HTML refers to an objective and a practice to create documents with HTML that contain only the author's intended meaning, without any reference to how this meaning is presented or conveyed. A classic example is the distinction between the emphasis element (<code>&lt;em&gt;</code>) and the italics element (<code>&lt;i&gt;</code>). Often the emphasis element is displayed in italics, so the presentation is typically the same. However, emphasizing something is different from listing the title of a book, for example, which may also be displayed in italics. In purely semantic HTML, a book title would use a different element than emphasized text uses (for example a <code>&lt;span&gt;</code>), because they are meaningfully different things.

The goal of semantic HTML requires two things of authors:

# To avoid the use of presentational markup (elements, attributes, and other entities).
# To use available markup to differentiate the meanings of phrases and structure in the document. So for example, the book title from above would need to have its own element and class specified, such as <code>&lt;cite class="booktitle"&gt;The Grapes of Wrath&lt;/cite&gt;.</code> Here, the <code>&lt;cite&gt;</code> element is used because it most closely matches the meaning of this phrase in the text. However, the <code>&lt;cite&gt;</code> element is not specific enough to this task, since we mean to cite specifically a book title as opposed to a newspaper article or an academic journal.

Semantic HTML also requires complementary specifications and software compliance with these specifications. Primarily, the development and proliferation of [[Cascading Style Sheets|CSS]] has led to increasing support for semantic HTML, because CSS provides designers with a rich language to alter the presentation of semantic-only documents. With the development of CSS, the need to include presentational properties in a document has virtually disappeared. With the advent and refinement of CSS and the increasing support for it in Web browsers, subsequent editions of HTML increasingly stress only using markup that suggests the semantic structure and phrasing of the document, like headings, paragraphs, quotes, and lists, instead of using markup which is written for visual purposes only, like <code>&lt;font&gt;</code>, <code>&lt;b&gt;</code> (bold), and <code>&lt;i&gt;</code> (italics). Some of these elements are not permitted in certain varieties of HTML, like HTML 4.01 Strict. CSS provides a way to separate document semantics from the content's presentation, by keeping everything relevant to presentation defined in a CSS file. See [[separation of style and content]].

Semantic HTML offers many advantages. First, it ensures consistency in style across elements that have the same meaning. Every heading, every quotation, every similar element receives the same presentation properties.

Second, semantic HTML frees authors from the need to concern themselves with presentation details. When writing the number two, for example, should it be written out in words ("two"), or should it be written as a numeral (2)? A semantic markup might enter something like <number>2</number> and leave presentation details to the stylesheet designers. Similarly, an author might wonder where to break out quotations into separate indented blocks of text: with purely semantic HTML, such details would be left up to stylesheet designers. Authors would simply indicate quotations when they occur in the text, and not concern themselves with presentation.

A third advantage is device independence and repurposing of documents. A semantic HTML document can be paired with any number of stylesheets to provide output to computer screens (through Web browsers), high-resolution printers, handheld devices, aural browsers or braille devices for those with visual impairments, and so on. To accomplish this, nothing needs to be changed in a well-coded semantic HTML document. Readily available stylesheets make this a simple matter of pairing a semantic HTML document with the appropriate stylesheets. (Of course, the stylesheet's selectors need to match the appropriate properties in the HTML document.)

Some aspects of authoring documents make separating semantics from style (in other words, meaning from presentation) difficult. Some elements are hybrids, using presentation in their very meaning. For example, a table displays content in a tabular form. Often such content conveys the meaning only when presented in this way. Repurposing a table for an aural device typically involves somehow presenting the table as an inherently visual element in an audible form. On the other hand, we frequently present lyrical songs&mdash;something inherently meant for audible presentation&mdash;and instead present them in textual form on a Web page. For these types of elements, the meaning is not so easily separated from their presentation. However, for a great many of the elements used and meanings conveyed in HTML, the translation is relatively smooth.

==Delivery of HTML==
HTML documents can be delivered by the same means as any other computer file; however, they are most often delivered in one of two forms: over [[HTTP]] servers and through e-mail.

===Publishing HTML with HTTP===
The [[World Wide Web]] is composed primarily of HTML documents transmitted from a [[Web server]] to a Web browser using the [[Hypertext Transfer Protocol]] (HTTP). However, HTTP can be used to serve images, sound, and other content in addition to HTML. To allow the Web browser to know how to handle the document it received, an indication of the [[file format]] of the document must be transmitted along with the document. This vital [[metadata]] includes the [[MIME]] type (<tt>text/html</tt> for HTML 4.01 and earlier, <tt>application/xhtml+xml</tt> for XHTML 1.0 and later) and the character encoding (see [[Character encodings in HTML]]).

In modern browsers, the MIME type that is sent with the HTML document affects how the document is interpreted. A document sent with an XHTML MIME type, or ''served as application/xhtml+xml'', is expected to be [[XML#Well-formed documents|well-formed]] XML, and a syntax error causes the browser to fail to render the document. The same document sent with an HTML MIME type, or ''served as text/html'', might be displayed successfully, since Web browsers are more lenient with HTML. However, XHTML parsed in this way is not considered either proper XHTML or HTML, but so-called [[tag soup]].

If the MIME type is not recognized as HTML, the Web browser should not attempt to render the document as HTML, even if the document is prefaced with a correct Document Type Declaration. Nevertheless, some Web browsers do examine the contents or URL of the document and attempt to infer the file type, despite this being forbidden by the HTTP 1.1 specification.

===HTML e-mail===
{{main|HTML e-mail}}
Most graphical [[e-mail]] clients allow the use of a subset of HTML (often ill-defined) to provide formatting and [[semantic web|semantic]] markup capabilities not available with [[plain text]], like emphasized text, block quotations for replies, and diagrams or mathematical formulas that could not easily be described otherwise. Many of these clients include both a [[GUI]] editor for composing HTML e-mail messages and a rendering engine for displaying received HTML messages. Use of HTML in e-mail is controversial because of compatibility issues, because it can be used in [[phishing]]/privacy attacks, because it can confuse [[E-Mail spam|spam]] filters, and because the message size is larger than plain text.

===Naming conventions===
The most common [[filename extension]] for [[computer file|files]] containing HTML is <tt>.html</tt>. A common abbreviation of this is <tt>.htm</tt>; it originates from older operating systems and file systems, such as the [[DOS]] versions from the 80s and early 90s and [[File Allocation Table|FAT]], which limit file extensions to three letters. Both forms are widely supported by browsers.

==Current flavors of HTML==
Since its inception, HTML and its associated protocols gained acceptance relatively quickly. However, no clear standards existed in the early years of the language. Though its creators originally conceived of HTML as a semantic language devoid of presentation details, practical uses pushed many presentational elements and attributes into the language, driven largely by the various browser vendors. The latest standards surrounding HTML reflect efforts to overcome the sometimes chaotic development of the language and to create a rational foundation for building both meaningful and well-presented documents. To return HTML to its role as a semantic language, the [[World Wide Web Consortium|W3C]] has developed style languages such as [[Cascading Style Sheets|CSS]] and [[Extensible Stylesheet Language|XSL]] to shoulder the burden of presentation. In conjunction, the HTML specification has slowly reined in the presentational elements.

There are two axes differentiating various flavors of HTML as currently specified: SGML-based HTML versus XML-based HTML (referred to as XHTML) on the one axis, and strict versus transitional (loose) versus frameset on the other axis.

===SGML-based versus XML-based HTML===
One difference in the latest HTML specifications lies in the distinction between the SGML-based specification and the XML-based specification. The XML-based specification is usually called XHTML to distinguish it clearly from the more traditional definition; however, the root element name continues to be 'html' even in the XHTML-specified HTML. The W3C intended XHTML 1.0 to be identical to HTML 4.01 except where limitations of XML over the more complex SGML require workarounds. Because XHTML and HTML are closely related, they are sometimes documented in parallel. In such circumstances, some authors conflate the two names as (X)HTML or X(HTML).<ref>See e.g., [[XHTML#Relationship to HTML]]</ref>

Like HTML 4.01, XHTML 1.0 has three sub-specifications: strict, loose, and frameset.

Aside from the different opening declarations for a document, the differences between an HTML 4.01 and XHTML 1.0 document&mdash;in each of the corresponding DTDs&mdash;are largely syntactic. The underlying syntax of HTML allows many shortcuts that XHTML does not, such as elements with optional opening or closing tags, and even EMPTY elements which must not have an end tag. By contrast, XHTML requires all elements to have an opening tag or a closing tag. XHTML, however, also introduces a new shortcut: an XHTML tag may be opened and closed within the same tag, by including a slash before the end of the tag like this: <code>&lt;br/&gt;</code>. 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.

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:
# '''The language for an element should be specified with a <code>lang</code> attribute rather than the XHTML <code>xml:lang</code> attribute.''' XHTML uses XML's built in language-defining functionality attribute.
# '''Remove the XML namespace (<code>xmlns=URI</code>).''' HTML has no facilities for namespaces.
# '''Change the document type declaration''' from XHTML 1.0 to HTML 4.01. (see [[#The Document Type Definition|DTD section]] for further explanation).
# If present, '''remove the XML declaration.''' (Typically this is: <code><?xml version="1.0" encoding="utf-8"?></code>).
# '''Ensure that the document’s MIME type is set to <code>text/html</code>.''' For both HTML and XHTML, this comes from the HTTP <code>Content-Type</code> header sent by the server.
# '''Change the XML empty-element syntax to an HTML style empty element''' (<code>&lt;br/&gt;</code> to <code>&lt;br&gt;</code>).

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 [http://www.w3.org/TR/xhtml1/#guidelines HTML Compatibility Guidelines]). The following steps can be applied to XHTML 1.0 documents only:
* Include both <code>xml:lang</code> and <code>lang</code> 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 <code>&lt;br&nbsp;/&gt;</code> instead of <code>&lt;br/&gt;</code>.
* Include explicit close tags for elements that permit content but are left empty (for example, <code>&lt;div&gt;</code><code>&lt;/div&gt;</code>, not <code>&lt;div /&gt;</code>).
* 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 <code>text/html</code> [[MIME type]]), or as XHTML (with an <code>application/xhtml+xml</code> or <code>application/xml</code> 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 ===
The latest SGML-based specification HTML 4.01 and the earliest XHTML version include three sub-specifications: Strict, Transitional (once called Loose), and Frameset. The Strict variant represents the standard proper, whereas the Transitional and Frameset variants were developed to assist in the transition from earlier versions of HTML (including HTML 3.2). The Transitional and Frameset variants allow for [[presentational markup]] whereas the Strict variant encourages the use of style sheets through its omission of most presentational markup.

The primary differences which make the Transitional variant more permissive than the Strict variant (the differences as the same in HTML 4 and XHTML 1.0) are:
* '''A looser content model'''
** Inline elements and plain text (#PCDATA) are allowed directly in: <code>body</code>, <code>blockquote</code>, <code>form</code>, <code>noscript</code> and <code>noframes</code>
* '''Presentation related elements'''
** underline (<code>u</code>)
** strike-through (<code>del</code>)
** <code>center</code>
** <code>font</code>
** <code>basefont</code>
* '''Presentation related attributes'''
** <code>background</code> and <code>bgcolor</code> attributes for <code>body</code> element.
** <code>align</code> attribute on <code>div</code>, <code>form</code>, <code>paragraph</code> (p), and heading (<code>h1</code>...<code>h6</code>) elements
** <code>align</code>, <code>noshade</code>, <code>size</code>, and <code>width</code> attributes on <code>hr</code> element
** <code>align</code>, <code>border</code>, <code>vspace</code>, and <code>hspace</code> attributes on <code>img</code> and <code>object</code> elements
** <code>align</code> attribute on <code>legend</code> and <code>caption</code> elements
** <code>align</code> and <code>bgcolor</code> on <code>table</code> element
** <code>nowrap</code>, <code>bgcolor</code>, <code>width</code>, <code>height</code> on <code>td</code> and <code>th</code> elements
** <code>bgcolor</code> attribute on <code>tr</code> element
** <code>clear</code> attribute on <code>br</code> element
** <code>compact</code> attribute on <code>dl</code>, <code>dir</code> and <code>menu</code> elements
** <code>type</code>, <code>compact</code>, and <code>start</code> attributes on <code>ol</code> and <code>ul</code> elements
** <code>type</code> and <code>value</code> attributes on <code>li</code> element
** <code>width</code> attribute on <code>pre</code> element
* '''Additional elements in Transitional specification'''
** <code>menu</code> list (no substitute, though unordered list is recommended; may return in XHTML 2.0 specification)
** <code>dir</code> list (no substitute, though unordered list is recommended)
** <code>isindex</code> (element requires server-side support and is typically added to documents server-side)
** <code>applet</code> (deprecated in favor of object element)
* '''The <code>language</code> attribute on script element''' (presumably redundant with <code>type</code> attribute, though this is maintained for legacy reasons).
* '''Frame related entities'''
** <code>frameset</code> element (used in place of body for frameset DTD)
** <code>frame</code> element
** <code>iframe</code>
** <code>noframes</code>
** <code>target</code> attribute on <code>anchor</code>, client-side image-map (<code>imagemap</code>), <code>link</code>, <code>form</code>, and <code>base</code> elements

===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:
<source lang="html4strict">
<html>
 <head>
  <title></title>
  <!-- other head elements -->
 </head>

 <!-- frameset replaces body -->
 <frameset>

  <!-- frame definitions -->
  <frame></frame>
  <!-- , ... -->

  <!-- optional: alternate page body for frames-incompatible user agents -->
  <noframes>
   <body></body>
  </noframes>

 </frameset>
</html>
</source>

=== Summary of flavors ===
As this list demonstrates, the loose flavors 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 clear 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 link]]s, [[transclusion]], [[source tracking]], [[fat link]]s, and more.<ref>{{cite web
|url=http://www.useit.com/alertbox/20050103.html
|title=Reviving Advanced Hypertext
|author=[[Jakob Nielsen (usability consultant)|Jakob Nielsen]]
|date=[[2005-01-03]]
|accessdate=2007-06-16}}</ref> Even some hypertext features that were in early versions of HTML have been ignored by most popular web browsers until recently, such as the [[Hyperlink|link]] element and in-browser Web page editing.

Sometimes Web services or browser manufacturers remedy these shortcomings. For instance, [[wiki]]s and [[content management system]]s allow surfers to edit the Web pages they visit.

==See also==
{{wikiversity}}
<div style="-moz-column-count:3; column-count:3;">
* [[Source Code]]
* [[Alt attribute]]
* [[Breadcrumb (navigation)]]
* [[Tim Berners-Lee]]
* [[Character encodings in HTML]]
* [[Comparison of document markup languages]]
* [[Comparison of layout engines (HTML)]]
* [[Comparison of layout engines (HTML 5)]]
* [[Cascading Style Sheets]]
* [[Dynamic HTML]]
* [[HTML 5]]
* [[HTML decimal character rendering]]
* [[HTML editor]]
* [[HTML element]]
* [[HTML scripting]]
* ''[[The HTML Sourcebook: The Complete Guide to HTML]]'' (historical reference from 1995)
* [[List of document markup languages]]
* [[List of XML and HTML character entity references]]
* [[Microformats]]
* [[Parsing]]
* [[Unicode and HTML]]
* [[Web colors]]
* [[XHTML]]
* [[Web Hypertext Application Technology Working Group]]
</div>

==References==
{{reflist|2}}

==External links==
{{wikibooks|HTML Programming}}
* [http://www.w3.org/TR/html401/ HTML 4.01, the last valid specification]
* [http://www.w3.org/MarkUp/Guide/ Dave Raggett's Introduction to HTML]
* [http://www.cs.tut.fi/~jkorpela/html/empty.html Empty elements in SGML, HTML, XML, and XHTML]
*[http://tunes.org/wiki/html_20special_20characters_20and_20symbols.html HTML special characters and symbols]

===HTML Markup Validators===
* [http://validator.w3.org/ W3C's Markup Validator]
* [http://kzone.host.sk/download/item4.htm Off-line HTML Validator v1.0] for Windows
* [http://validator.nu/ Validator.nu]

=== Standard HTML specifications ===
* Refer to the [[#Version history of the standard|HTML version history section]]

===Other specifications===
* [http://whatwg.org/specs/web-apps/current-work/ Web Applications 1.0] A specification generally referred to as "HTML 5". The Web Hypertext Application Technology working group are an independent initiative who cooperate with the W3C.

{{W3C Standards}}

[[Category:HTML|HTML]]
[[Category:Computing acronyms]]
[[Category:Markup languages]]
[[Category:Technical communication]]
[[Category:World Wide Web Consortium standards]]

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[[tr:Zengin metin işaretleme dili]]
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