HTML, which stands for Hyper Text 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 etc as well as for 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 include or can load scripts in languages such as JavaScript which affect the behavior of HTML processors like Web browsers; and 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.[1]Contents [hide]
1 History
1.1 Origins
1.2 First specifications
1.3 Version history of the standard
1.3.1 HTML version timeline
1.3.2 HTML draft version timeline
1.3.3 XHTML versions
2 Markup
2.1 Elements
2.1.1 Attributes
2.2 Character and entity references
2.3 Data types
2.4 Document type declaration
3 Semantic HTML
4 Delivery
4.1 HTTP
4.2 HTML e-mail
4.3 Naming conventions
4.4 HTML Application
5 Current variations
5.1 SGML-based versus XML-based HTML
5.2 Transitional versus strict
5.3 Frameset versus transitional
5.4 Summary of specification versions
6 Hypertext features not in HTML
7 Additional info
8 See also
9 References
10 External links
10.1 HTML tutorials
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History
[edit]
Origins
Tim Berners-Lee
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.[2] 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,[3] from 1990 he lists[4] "some of the many areas in which hypertext is used", and puts an encyclopedia first.
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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.[5][6] 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.[7]
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 point effects, and 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, and 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.[8] 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.[9] 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.[10]
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.[9] Published as Request for Comments 1866, HTML 2.0 included ideas from the HTML and HTML+ drafts.[11] The 2.0 designation was intended to distinguish the new edition from previous drafts.[12]
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).[13] 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.
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Version history of the standardHTML
HTML and HTML5
Dynamic HTML
XHTML
XHTML Mobile Profile and C-HTML
Character encodings
Font family
HTML editor
HTML element
HTML scripting
Layout engine
Quirks mode
Style sheets
Unicode and HTML
W3C
Web colors
Comparison of
document markup languages
web browsers
layout engines for
HTML
HTML5
Non-standard HTML
XHTML
This box: view • talk • edit
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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[14] 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.[15]
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.[13] A markup for mathematical formulas similar to that in HTML wasn't standardized until 14 months later in MathML.
December 1997
HTML 4.0[16] 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",[17] 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.[18]
April 1998
HTML 4.0[19] was reissued with minor edits without incrementing the version number.
December 1999
HTML 4.01[20] 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[21][22] ("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).[21]
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.
[edit]
HTML draft version timeline
October 1991
HTML Tags,[5] an informal CERN document listing twelve HTML tags, was first mentioned in public.
July 1992
First informal draft of the HTML DTD,[1] with six 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[23] 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[2] one month later, followed by six further drafts published by IETF itself[3] 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[24] 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.[25]
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.[13] 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.[13] (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."[13] 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."[13]
January 2008
HTML 5[26] was published as a Working Draft 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.[27]
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XHTML versions
Main article: XHTML
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,[28] 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,[29] 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 Modularization of XHTML, which was published April 10, 2001 as a W3C Recommendation.
XHTML 2.0,[30] is still a W3C Working Draft. W3C announched that the XHTML 2 group will stop work by end of 2009[31]. There will be 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.[32]
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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, 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[4].
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:
Hello HTML
Hello World!
This Document Type Declaration is for HTML 5.
If the declaration is not included, most browsers will render using "quirks mode."[33]
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Elements
Main article: HTML element
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:
content to be rendered
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:
content to be rendered
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:
The title
Paragraph Partition:
Paragraph 1
Paragraph 2
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:
This
is a paragraph
with
line breaks
Annotation:
Annotations can help to understand the coding and do not display in the webpage.
There are several types of markup elements used in HTML.
Structural markup describes the purpose of text. For example,
Golf
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
boldface 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
bold and
italic, there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely
strong emphasis and
emphasis 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:
Wikipedia. In addition, the href attribute must be set to a valid URL. For example, the HTML markup,
Wikipedia, will render the word "Wikipedia" as a hyperlink. An example to render an image as a hyperlink is:
.
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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).[34][35] Leaving attribute values unquoted is considered unsafe.[36] 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[5] (like the ismap attribute for the img element[37]).
Most elements can take any of several common attributes:
The id 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. 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 http://en.wikipedia.org/wiki/HTML#Attributes
The class attribute provides a way of classifying similar elements. This can be used for semantic or presentation purposes. Semantically, for example, classes are used in microformats. Presentationally, for example, an HTML document might use the designation class="notation" 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 style non-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s id or class attributes to select the element with a stylesheet, though sometimes this can be too cumbersome for a simple and specific or ad hoc application of styled properties.
The title 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 abbreviation element, abbr, can be used to demonstrate these various attributes:
HTML
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 attributes lang and dir.
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Character and entity references
See also: List of XML and HTML character 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 e (é), a character typically found only on Western European keyboards, can be written in any HTML document as the entity reference é or as the numeric references é or é. The characters comprising those references (that is, the &, the ;, the letters in eacute, and so on) are available on all keyboards and are supported in all character encodings, whereas the literal é is not. [edit] 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. [edit] 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 the quirks mode. The original purpose of the doctype was to enable validation based on Document Type Definition (DTD) with SGML tools. 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 read the DTD, however. HTML 5 validation is not DTD-based, so in HTML 5 the doctype does not refer to a DTD. An example of an HTML 4 doctype:
This declaration references the Strict DTD of HTML 4.01, which does not have presentational elements like
, leaving formatting to Cascading Style Sheets and the span and div 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 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.
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Semantic HTML
Main article: 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,[38] but has also included presentational markup such as , and tags. There are also the semantically neutral span and div tags. Since the late 1990s when Cascading Style Sheets were beginning to work in most browsers, web authors have been encouraged to avoid the use of presentational HTML markup with a view to the separation of presentation and content.[39]
In a 2001 discussion of the Semantic Web, Tim Berners-Lee and others gave examples of ways in which intelligent software 'agents' may one day automatically trawl the Web and find, filter and correlate previously unrelated, published facts for the benefit of human users.[40] Such agents are not commonplace even now, but some of the ideas of Web 2.0, mashups and price comparison websites may be coming close. The main difference between these web application hybrids and Berners-Lee's semantic agents lies in the fact that the current aggregation and hybridisation of information is usually designed in by web developers, who already know the web locations and the API semantics of the specific data they wish to mash, compare and combine.
An important type of web agent that does trawl and read web pages automatically, without prior knowledge of what it might find, is the Web crawler or search-engine spider. These software agents are dependent on the semantic clarity of web pages they find as they use various techniques and algorithms to read and index millions of web pages a day and provide web users with search facilities without which the World Wide Web would be only a fraction of its current usefulness.
In order for search-engine spiders to be able to rate the significance of pieces of text they find in HTML documents, and also for those creating mashups and other hybrids, as well as for more automated agents as they are developed, the semantic structures that exist in HTML need to be widely and uniformly applied to bring out the meaning of published text.[41]
Presentational markup tags are deprecated in current HTML and XHTML recommendations and are illegal in HTML 5.
Good semantic HTML also improves the accessibility of web documents (see also Web Content Accessibility Guidelines). For example, when a screen reader or audio browser can correctly ascertain the structure of a document, it will not waste the visually impaired user's time by reading out repeated or irrelevant information when it has been marked up correctly.
[edit]
Delivery
HTML documents can be delivered by the same means as any other computer file; however, they are most often delivered either by HTTP from a Web server or by e-mail.
[edit]
HTTP
The World Wide Web is composed primarily of HTML documents transmitted from Web servers to Web browsers using the Hypertext Transfer Protocol (HTTP). However, HTTP is used to serve images, sound, and other content in addition to HTML. To allow the Web browser to know how to handle each document it receives, other information is transmitted along with the document. This meta data usually includes the MIME type (e.g. text/html or application/xhtml+xml) and the character encoding (see Character encoding in HTML).
In modern browsers, the MIME type that is sent with the HTML document may affect how the document is initially interpreted. A document sent with the XHTML MIME type is expected to be well-formed XML, and syntax errors may cause the browser to fail to render it. The same document sent with the HTML MIME type might be displayed successfully, since some browsers are more lenient with HTML.
The W3C recommendations state that XHTML 1.0 documents that follow guidelines set forth in the recommendation's Appendix C may be labeled with either MIME Type.[42] The current XHTML 1.1 Working Draft also states that XHTML 1.1 documents should[43] be labeled with either MIME type.[44]
[edit]
HTML e-mail
Main article: HTML e-mail
Most graphical e-mail clients allow the use of a subset of HTML (often ill-defined) to provide formatting and semantic markup not available with plain text. This may include typographic information like coloured headings, emphasized and quoted text, inline images and diagrams. Many such clients include both a GUI editor for composing HTML e-mail messages and a rendering engine for displaying them. Use of HTML in e-mail is controversial because of compatibility issues, because it can help disguise phishing attacks, because it can confuse spam filters and because the message size is larger than plain text.
[edit]
Naming conventions
The most common filename extension for files containing HTML is .html. A common abbreviation of this is .htm, which originated because some early operating systems and file systems, such as DOS and FAT, limited file extensions to three letters.
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HTML Application
Main article: HTML Application
An HTML Application (HTA; file extension ".hta") is a Microsoft Windows application that uses HTML and Dynamic HTML in a browser to provide the application's graphical interface. A regular HTML file is confined to the security model of the web browser, communicating only to web servers and manipulating only webpage objects and site cookies. An HTA runs as a fully trusted application and therefore has more privileges, like creation/editing/removal of files and Windows Registry entries. Because they operate outside the browser's security model, HTAs cannot be executed via HTTP, but must be downloaded (just like an EXE file) and executed from local file system.
[edit]
Current variations
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[5], 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[6] 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 W3C has developed style languages such as CSS and 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 variations of HTML as currently specified: SGML-based HTML versus XML-based HTML (referred to as XHTML) on one axis, and strict versus transitional (loose) versus frameset on the other axis.
[edit]
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).[45]
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—in each of the corresponding DTDs—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:
. 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:
.[46]
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 lang attribute rather than the XHTML xml:lang attribute. XHTML uses XML's built in language-defining functionality attribute.
Remove the XML namespace (xmlns=URI). HTML has no facilities for namespaces.
Change the document type declaration from XHTML 1.0 to HTML 4.01. (see DTD section for further explanation).
If present, remove the XML declaration. (Typically this is: ).
Ensure that the document’s MIME type is set to text/html. For both HTML and XHTML, this comes from the HTTP Content-Type header sent by the server.
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 xml:lang and lang 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 text/html MIME type), or as XHTML (with an application/xhtml+xml or application/xml 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.
[edit]
Transitional versus strict This section may be confusing or unclear to readers. Please help clarify the section; suggestions may be found on the talk page. (February 2009)
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 are the same in HTML 4 and XHTML 1.0) are:
A looser content model
Inline elements and plain text (#PCDATA) are allowed directly in: body, blockquote, form, noscript and noframes
Presentation related elements
underline (u)
strike-through (s)
center
font
basefont
Presentation related attributes
background and bgcolor attributes for body element.
align attribute on div, form, paragraph (p), and heading (h1...h6) elements
align, noshade, size, and width attributes on hr element
align, border, vspace, and hspace attributes on img and object elements
align attribute on legend and caption elements
align and bgcolor on table element
nowrap, bgcolor, width, height on td and th elements
bgcolor attribute on tr element
clear attribute on br element
compact attribute on dl, dir and menu elements
type, compact, and start attributes on ol and ul elements
type and value attributes on li element
width attribute on pre element
Additional elements in Transitional specification
menu list (no substitute, though unordered list is recommended; may return in XHTML 2.0 specification)
dir list (no substitute, though unordered list is recommended)
isindex (element requires server-side support and is typically added to documents server-side)
applet (deprecated in favor of object element)
The language attribute on script element (presumably redundant with type attribute, though this is maintained for legacy reasons).
Frame related entities
frameset element (used in place of body for frameset DTD)
frame element
iframe
noframes
target attribute on anchor, client-side image-map (imagemap), link, form, and base elements
[edit]
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 frameset replacing body, containing frame elements, and optionally noframes, with a body.
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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—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.
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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 more.[47] 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.
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Additional info
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, and the editor renders this as an HTML document. Although this is probably the quickest way to begin creating HTML pages, it is also common to write HTML using a simple text editor. See the list of HTML editors.
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See also
Breadcrumb (navigation)
HTML decimal character rendering
HTML element
JHTML
List of computer standards
List of document markup languages
Microformat
The HTML Sourcebook: The Complete Guide to HTML (historical reference from 1995)
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References
^ HTML 4 — Conformance: requirements and recommendations
^ Tim Berners-Lee, "Information Management: A Proposal." CERN (March 1989, May 1990). W3.org
^ Tim Berners-Lee, "Design Issues"
^ Tim Berners-Lee, "Design Issues"
^ a b c "Tags used in HTML". World Wide Web Consortium. November 3, 1992. Retrieved November 16, 2008.
^ "First mention of HTML Tags on the www-talk mailing list". World Wide Web Consortium. October 29, 1991. Retrieved April 8, 2007.
^ "Index of elements in HTML 4". World Wide Web Consortium. December 24, 1999. Retrieved April 8, 2007.
^ Tim Berners-Lee (December 9, 1991). "Re: SGML/HTML docs, X Browser (archived www-talk mailing list post)". Retrieved June 16, 2007. "SGML is very general. HTML is a specific application of the SGML basic syntax applied to hypertext documents with simple structure."
^ a b Raymond, Eric. "IETF and the RFC Standards Process". The Art of Unix Programming. http://www.faqs.org/docs/artu/ietf_process.html. "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."
^ "HTML+ Internet-Draft - Abstract". "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."
^ "RFC 1866: Hypertext Markup Language - 2.0 - Acknowledgments". Internet Engineering Task Force. September 22, 2005. Retrieved June 16, 2007. "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."
^ "RFC 1866: Hypertext Markup Language - 2.0 - Introduction". Internet Engineering Task Force. September 22, 2005. Retrieved June 16, 2007. "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."
^ a b c d e f Raggett, Dave (1998). Raggett on HTML 4. Retrieved July 9, 2007.
^ "HTML 3.2 Reference Specification". World Wide Web Consortium. January 14, 1997. Retrieved November 16, 2008.
^ "IETF HTML WG". Retrieved June 16, 2007. "Note: This working group is closed"
^ "HTML 4.0 Specification". World Wide Web Consortium. December 18,1997. Retrieved November 16, 2008.
^ Arnoud Engelfriet. "Introduction to Wilbur". Web Design Group. Retrieved June 16, 2007.
^ "HTML 4 - 4 Conformance: requirements and recommendations". Retrieved December 30, 2009.
^ "HTML 4.0 Specification". World Wide Web Consortium. April 24, 1998. Retrieved November 16, 2008.
^ "HTML 4.01 Specification". World Wide Web Consortium. December 24, 1999. Retrieved November 16, 2008.
^ a b ISO (2000). "ISO/IEC 15445:2000 - Information technology -- Document description and processing languages -- HyperText Markup Language (HTML)". Retrieved December 26, 2009.
^ CS.TCD.ie
^ Hypertext Markup Language: A Representation of Textual Information and MetaInformation for Retrieval and Interchange
^ "HTML 3.0 Draft (Expired!) Materials". World Wide Web Consortium. December 21, 1995. Retrieved November 16, 2008.
^ "HyperText Markup Language Specification Version 3.0". Retrieved June 16, 2007.
^ "HTML 5". World Wide Web Consortium. June 10, 2008. Retrieved November 16, 2008.
^ "HTML 5, one vocabulary, two serializations". Retrieved February 25, 2009.
^ "XHTML 1.0: The Extensible HyperText Markup Language (Second Edition)". World Wide Web Consortium. January 26, 2000. Retrieved November 16, 2008.
^ "XHTML 1.1 - Module-based XHTML - Second Edition". World Wide Web Consortium. February 16, 2007. Retrieved November 16, 2008.
^ "XHTM 2.0". World Wide Web Consortium. July 26, 2006. Retrieved November 16, 2008.
^ "XHTML 2 Working Group Expected to Stop Work End of 2009, W3C to Increase Resources on HTML 5". World Wide Web Consortium. July 17, 2009. Retrieved November 16, 2008.
^ "HTML 5". World Wide Web Consortium. October 24, 2008. Retrieved November 16, 2008.
^ Activating Browser Modes with Doctype
^ "On SGML and HTML". World Wide Web Consortium. Retrieved November 16, 2008.
^ "XHTML 1.0 - Differences with HTML 4". World Wide Web Consortium. Retrieved November 16, 2008.
^ Korpela, Jukka (July 6, 1998). "Why attribute values should always be quoted in HTML". Cs.tut.fi. Retrieved November 16, 2008.
^ "Objects, Images, and Applets in HTML documents". World Wide Web Consortium. December 24, 1999. Retrieved November 16, 2008.
^ Berners-Lee, Tim; Fischetti, Mark (2000). Weaving the Web: The Original Design and Ultimate Destiny of the World Wide Web by Its Inventor. San Francisco: Harper. ISBN 978-0-06-251587-X.
^ Raggett, Dave (2002). "Adding a touch of style". W3C. Retrieved October 2, 2009. This article notes that presentational HTML markup may be useful when targeting browsers "before Netscape 4.0 and Internet Explorer 4.0". See the list of web browsers to confirm that these were both released in 1997.
^ Tim Berners-Lee, James Hendler and Ora Lassila (2001). "The Semantic Web". Scientific American. Retrieved October 2, 2009.
^ Nigel Shadbolt, Wendy Hall and Tim Berners-Lee (2006). "The Semantic Web Revisited". IEEE Intelligent Systems. Retrieved October 2, 2009.
^ "XHTML 1.0 The Extensible HyperText Markup Language (Second Edition)". World Wide Web Consortium. 2000, revised 2002. Retrieved December 7, 2008. "XHTML Documents which follow the guidelines set forth in Appendix C, "HTML Compatibility Guidelines" may be labeled with the Internet Media Type "text/html" [RFC2854], as they are compatible with most HTML browsers. Those documents, and any other document conforming to this specification, may also be labeled with the Internet Media Type "application/xhtml+xml" as defined in [RFC3236]."
^ "RFC 2119: Key words for use in RFCs to Indicate Requirement Levels". Harvard University. 1997. Retrieved December 7, 2008. "3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course."
^ "XHTML 1.1 - Module-based XHTML - Second Edition". World Wide Web Consortium. 2007. Retrieved December 7, 2008. "XHTML 1.1 documents SHOULD be labeled with the Internet Media Type text/html as defined in [RFC2854] or application/xhtml+xml as defined in [RFC3236]."
^ See e.g., XHTML#Relationship to HTML
^ Freeman, E (2005). Head First HTML. O'Reilly.
^ Jakob Nielsen (January 3, 2005). "Reviving Advanced Hypertext". Retrieved June 16, 2007.
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External links Wikibooks has a book on the topic of
HyperText Markup Language
Wikiversity has learning materials about HTML
Wikiversity has learning materials about HTML Challenges
HTML 4.01, the most recent finished specification (1999)
HTML 5, the upcoming version of HTML
Dave Raggett's Introduction to HTML
Empty elements in SGML, HTML, XML, and XHTML
(X)HTML Entities or Special Characters simple Reference
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HTML tutorials
HTML Dog
Your HTML Source[show]
v • d • e
Document markup languages (list)
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v • d • e
Standards of the World Wide Web Consortium
Recommendations Canonical XML · CDF · CSS · DOM · HTML · MathML · OWL · P3P · PLS · RDF · RDF Schema · SISR · SMIL · SOAP · SRGS · SSML · SVG · SPARQL · Timed Text · VoiceXML · WSDL · XForms · XHTML · XLink · XML · XML Base · XML Encryption · XML Events · XML Information Set · XML Schema · XML Signature · XPath · XPointer · XQuery · XSL · XSL-FO · XSLT
Notes XAdES · XHTML+SMIL
Working Drafts CCXML · CURIE · HTML5 · InkML · WICD · XFDL · XFrames · XBL · XHTML+MathML+SVG · XMLHttpRequest · XProc
Guidelines Web Content Accessibility Guidelines
Deprecated C-HTML · HDML · JSSS · PGML · VML
W3C Device Description Working Group
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v • d • e
ISO Standards
Categories: HTML | Computing acronyms | Markup languages | Technical communication | World Wide Web Consortium standards | Internet terminology
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