HTML

HTML, short for Hypertext Markup Language, is the predominant markup language for the creation of web pages. It provides a means to describe the structure of text-based information in a document — by denoting certain text as 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 labels (known as tags), surrounded by less-than (<) and greater-than signs (>). HTML can also describe, to some degree, the appearance and semantics of a document, and can include embedded scripting language code which can affect the behavior of browsers and other HTML processors.

HTML is also often used to refer to content of the 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).

Basic features

 * Structured text web pages, with visual formatting of:
 * chapter and section headings,
 * paragraphs and text markup such as italics and bold to stress parts of text,
 * unnumbered and numbered lists,
 * tables;
 * embedding of visible raster images into the text flow;
 * links, which provide access to other web pages on the World Wide Web.

Various variants of HTML, integrated with CSS and DOM access through EcmaScript (JavaScript and similar), allow for advanced dynamic web page design.

Definition of HTML
HTML stands for HyperText Markup Language.


 * 1) Hypertext is ordinary text that has been dressed up with extra features, such as formatting, images, multimedia, and links to other resources.
 * 2) Markup is the process of taking ordinary text and adding extra symbols. Each of the symbols used for markup in HTML is a command that tells a browser how to display the text.

Origins
In 1980, physicist Tim Berners-Lee, who was an independent contractor at CERN, proposed and prototyped ENQUIRE, a hypertext system for CERN researchers to use to 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, which was accepted by CERN.

At the time, HTML was not a specification, but a collection of loosely defined elements to solve an immediate problem: the communication and dissemination of ongoing research between Berners-Lee and his colleagues. Rather than reusing existing hypertext systems which were too commercial, too platform-specific, or too complicated for authors, Berners-Lee developed his own, relatively simple system. His original browsing software, a client called "WorldWideWeb", interacting with a server called "httpd", was written in November 1990 on a NeXTcube workstation, using the NEXTSTEP development environment. It tied together his inventions of a document identification system (which later evolved into the URI standard), a protocol (HTTP) for transmitting documents over a TCP/IP network, and a document annotation convention he later referred to as Hypertext Markup Language (HTML). HTML essentially grafted hypertext capability onto a homegrown SGML-like markup language, and Berners-Lee's software allowed a computer user to view and navigate between HTML documents accessed via the Internet. His solution later combined with the emerging international and public Internet to garner worldwide attention.

First specifications
The first publicly available description of HTML was a document called "HTML Tags", first mentioned on the Internet by Berners-Lee in late 1991. It describes 22 elements comprising the initial, relatively simple design of HTML. Thirteen of these elements still exist in HTML 4.

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 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. The draft expired after six months, but was notable for its acknowledgment of the NCSA Mosaic browser's custom tag for embedding in-line images, reflecting the IETF's philosophy of basing standards on successful prototypes. Similarly, Dave Raggett's competing Internet-Draft, "HTML+ (Hypertext Markup Format)", from late 1993, suggested standardizing already-implemented features like tables and fill-out forms.

After the HTML and HTML+ drafts expired in early 1994, the IETF created an HTML Working Group, which in 1995 completed "HTML 2.0", the first HTML specification intended to be treated as a standard against which future implementations should be based. Published as Request for Comments 1866, HTML 2.0 included ideas from the HTML and HTML+ drafts. There was no "HTML 1.0"; the 2.0 designation was intended to distinguish the new edition from previous drafts.

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

Increasing strictness
Early HTML syntax rules and processing requirements were ambiguous or, by design, lenient; when confronted with unfamiliar or poorly-authored markup, Web browsers commonly made assumptions about intent and proceeded with rendering of the document. This helped speed the adoption of HTML by document authors. Over time, as the use of authoring tools increased and more consistent browser behavior was demanded, the trend in the official standards has been to create an increasingly strict language syntax, demanding more precise code. Most browsers, however, continue to forgivingly render documents that are far from valid HTML.

Since the publication of HTML 4.0 in late 1997, the W3C's HTML Working Group focused increasingly — and from 2002 through 2006, exclusively — on the development of XHTML, an XML-based counterpart to HTML that is described on one W3C web page as HTML's "successor". In 2007, the old HTML Working Group was renamed to XHTML2 Working Group and a new HTML Working Group was chartered to continue the development of HTML.

XHTML is a reformulation of HTML as an XML vocabulary, and can be mixed with other XML vocabularies such as SVG and MathML. XHTML served using the media type for HTML,, has been embraced by many web standards advocates in preference to HTML. XHTML is routinely characterized by mass-media publications for both general and technical audiences as the newest "version" of HTML, but W3C publications, as of 2007, do not make such a claim. Neither HTML 3.2 nor HTML 4.01 have been explicitly rescinded, deprecated, or superseded by any W3C publications; as of 2007, they continue to be listed alongside XHTML as current Recommendations in the W3C's primary publication indices.

New development
In November 2006, the W3C published a new draft charter indicating its intent to resume development of HTML in a manner that unifies HTML and XHTML 1.x, allowing this hybrid language to manifest in both an XML format and a "classic HTML" format that is not strictly SGML-based. Among other things, it is planned that the new specification, to be refined and released from 2007 to 2010, will include conformance and parsing requirements, DOM APIs, and new widgets and APIs. The group also intends to publish test suites and validation tools. The new HTML working group was rechartered in March 2007 The old HTML working group was rechartered and renamed to XHTML2 WG.

On 2007-04-10, the Mozilla Foundation, Apple Computer and Opera Software proposed that the new HTML working group of the W3C adopt the WHATWG’s HTML 5 as the starting point of its work and name its future deliverable “HTML 5”. On 2007-05-09, the new HTML working group resolved to do that.

HTML versions
July, 1993: Hypertext Markup Language,was published at IETF working draft (that is: not a standard – yet).

November, 1995: 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) in January 1997;

ultimately all were declared obsolete/historic by RFC 2854 in June 2000.

An HTML 3.0 standard was proposed to the IETF by Dave Raggett and the newly formed W3C in April 1995. It proposed 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 math elements. Even though it was designed to be compatible with HTML 2.0, it was too complex at the time to be implemented. Browser vendors opted to support only parts of the proposal, but implemented other markup constructs that they wanted to be incorporated into the standard. When the draft expired in September 1995, work in this direction was discontinued due to lack of browser support. HTML 3.1 was never officially proposed, and the next standard proposal was HTML 3.2 (code-named "Wilbur"), which dropped the majority of the new features in HTML 3.0 and instead adopted many browser-specific element types and attributes that had been created for the Netscape and Mosaic web browsers.

January 14, 1997: HTML 3.2, published as a W3C Recommendation.

HTML 3.2 was never submitted to the IETF, whose HTML Working Group closed in September 1996; it was instead published as one of the W3C's first "Recommendations" in early 1997. Math support as proposed by HTML 3.0 finally came about years later with a different standard, MathML.

December 18, 1997: 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 frame related elements are allowed;

HTML 4.0 (initially code-named "Cougar") likewise adopted many browser-specific element types and attributes, but at the same time began to try to "clean up" the standard by marking some of them as deprecated, and suggesting they not be used. Minor editorial revisions to the HTML 4.0 specification were published as HTML 4.01.

December 24, 1999: HTML 4.01, published as a W3C Recommendation. It offers the same three flavors as HTML 4.0, and its last errata was published May 12, 2001.

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

May 15, 2000: ISO/IEC 15445:2000 ("ISO HTML", based on HTML 4.01 Strict), published as an ISO/IEC international standard.

HTML 5 is still an Editor’s Draft, and not endorsed by W3C yet.

XHTML versions
XHTML is a separate language that began as a reformulation of HTML 4.01 using XML 1.0. It continues to be developed:


 * XHTML 1.0, published January 26, 2000 as a W3C Recommendation, later revised and republished August 1, 2002. It offers the same three flavors as HTML 4.0 and 4.01, reformulated in XML, with minor restrictions.
 * XHTML 1.1, published May 31, 2001 as a W3C Recommendation. It is based on XHTML 1.0 Strict, but includes minor changes, can be customized, and is reformulated using modules from Modularization of XHTML, which was published April 10, 2001 as a W3C Recommendation.
 * 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.
 * XHTML5, which is an update to XHTML 1.x, is being defined alongside HTML5 in the HTML 5 draft.

HTML markup
HTML markup consists of several types of entities, including: elements, attributes, data types and character references.

The Document Type Definition
In order to enable Document Type Definition (DTD)-based validation with SGML tools and in order to avoid the Quirks mode in browsers, all HTML documents should start with a Document Type Declaration (informally, a "DOCTYPE"). The DTD contains machine readable grammar specifying the permitted and prohibited content for a document conforming to such a DTD. Browsers do not direct to the page in the DTD, however. Browsers only look at the doctype in order to decide the layout mode. Not all doctypes trigger the Standards layout mode avoiding the Quirks mode. For example: 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

and

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 the Standards layout mode for CSS as opposed to the 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.

Elements

 * See HTML elements 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 label (e.g. ) and an end label (e.g.  ). The element's attributes are contained in the start label and content is located between the labels (e.g.  ). Some elements, such as  , do not have any content and so need no closing label. Listed below are several types of markup elements used in HTML.

Structural markup describes the purpose of text. For example,  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. Further styling should be done with Cascading Style Sheets (CSS).

Presentational markup describes the appearance of the text, regardless of its function. For example   indicates that visual output devices should render "boldface" in bold text, but gives no indication what devices which are unable to do this (such as aural devices that read the text aloud) should do. In the case of both   and   there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely and  respectively. It is easier to see how an aural user agent should interpret the latter two elements. However, they are not equivalent to their presentational counterparts: it would be undesirable for a screen-reader to emphasize the name of a book, for instance, but on a screen such a name would be italicized. Most presentational markup elements have become deprecated under the HTML 4.0 specification, in favor of CSS based style design.

Hypertext markup 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:. However, the  attribute must also be set to a valid URL so for example the HTML code, , will render the word " wiki.gis.com " as a hyperlink.

Attributes
The attributes of an element are name-value pairs, separated by "=", and written within the start label of an element, after the element's name. The value should be enclosed in single or double quotes, although values consisting of certain characters can be left unquoted in HTML (but not XHTML). Leaving attribute values unquoted is considered unsafe.

Most elements take any of several common attributes:,  ,   and. Most also take language-related attributes:  and.

The  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  attribute provides a way of classifying similar elements for presentation purposes. For example, an HTML (or a set of documents) document may use the designation  to indicate that all elements with this class value are all subordinate to the main text of the document (or documents). Such notation classes of elements might be gathered together and presented as footnotes on a page, rather than appearing in the place where they appear in the source HTML.

An author may use the  non-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s son-  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  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 can be used to demonstrate these various non-attributes, which displays as HTML (pointing the cursor at the abbreviation should display the title text in most browsers).

Other markup
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 equivalent are considered equivalent and are rendered identically.

The ability to "escape" characters in this way allows for the characters "&lt;" and "&amp;" (when written as  and , respectively) to be interpreted as character data, rather than markup. For example, a literal "&lt;" normally indicates the start of a label, and "&amp;" normally indicates the start of a character entity reference or numeric character reference; writing it as "&amp;amp;" or "&amp;#38;" allows "&amp;" 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 "&amp;quot;" or "&amp;#22;" when it appears within in 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, "é", a character typically found only on Western European keyboards, can be written in any HTML document as the entity reference  or as the numeric references   or. The characters comprising those references (that is, the "&amp;", 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.

HTML also 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.

Semantic HTML
There is no official specification called "Semantic HTML", though the strict flavors of HTML discussed 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 and the italics element. 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 separate element than emphasized text uses (for example a ), because they are each meaningfully different things.

The goal of semantic HTML requires two things of authors:

1) to avoid the use of presentational markup (elements, attributes and other entities); 2) the use of 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  Here, the    element is used, because it most closely matches the meaning of this phrase in the text. However, the  element is not specific enough to this task because we mean to cite specifically a book title as opposed to a newspaper article or a particular academic journal.

Semantic HTML also requires complementary specifications and software compliance with these specifications. Primarily, the development and proliferation of 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,   (bold), and   (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 mark, 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 2 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 this content only conveys the meaning 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, HTML documents are most often delivered in one of the following two forms: Over HTTP servers and through email.

Publishing HTML with HTTP
The World Wide Web is primarily composed 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 (text/html for HTML 4.01 and earlier, application/xhtml+xml 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 well-formed XML and a syntax error causes the browser to fail to render the document. The same document sent with a HTML MIME type, or served as text/html, might get displayed since web browsers are more lenient with HTML. However, XHTML parsed this way is not considered either proper XHTML nor 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
Most graphical e-mail clients allow the use of a subset of HTML (often ill-defined) to provide formatting and semantic markup capabilities not available with plain text, like emphasized text, block quotations for replies, and diagrams or mathematical formulas that couldn't easily be described otherwise. Many of these clients include both a GUI editor for composing HTML e-mails and a rendering engine for displaying received HTML e-mails. Use of HTML in e-mail is controversial due to compatibility issues, because it can be used in phishing/privacy attacks, because it can confuse spam filters, and because the message size is larger than plain text.

The tag for email is:

Naming conventions
The most common filename extension for files containing HTML is .html. A common abbreviation of this is .htm; it originates from older operating systems and file systems, such as the DOS versions from the 80's and early 90's and 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 to build 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 within the specification.

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.

Traditional 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 clearly distinguish it 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.

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; is 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:  attribute rather than the XHTML   attribute''' XHTML uses XML's built in language defining functionality attribute.
 * 1) Remove the XML namespace . HTML has no facilities for namespaces.
 * 2) Change the document type declaration from XHTML 1.0 to HTML 4.01. (see DTD section for further explanation).
 * 3) If present, remove the XML declaration (Typically this is:  ).
 * 4) Ensure the document’s mime type is set to text/html In an XHTML document, this may come from the HTTP header sent by the server or from the XML declaration at the start of the document.  In an HTML document, this may come from the HTTP header sent by the server or a   element within the HTML.
 * 5) Change the XML empty element short-cut 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 labels within an HTML document rather than remembering which labels 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:


 * Including both  and   attributes on any elements assigning language.
 * Using the self-closing tag only for elements specified as empty in HTML
 * Including an extra space in self-closing labels: for example  instead of
 * Including explicit close labels for elements that permit content but are left empty (for example, " ", not " " )
 * Omit the XML declaration

Note that 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 which are XHTML 1.0 and have been made compatible in this way, the W3C permits them to the 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.

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:,  ,  ,   and
 * Presentation related elements
 * underline
 * strike-through ( and  )
 * Presentation related attributes
 * and  attributes for   element.
 * attribute on,  ,   (p), and heading ( ... ) elements
 * ,,  , and   attributes on   element
 * ,,  , and   attributes on   and   elements
 * attribute on  and   elements
 * and  on   element
 * ,,  ,   on   and   elements
 * attribute on  element
 * attribute on  element
 * attribute on,   and   elements
 * ,, and   attributes on   and   elements
 * and  attributes on   element
 * attribute on  element
 * Additional elements in Transitional specification
 * list (no substitute, though unordered list is recommended; may return in XHTML 2.0 specification)
 * list (no substitute, though unordered list is recommended)
 * (element requires server-side support and is typically added to documents server-side)
 * (deprecated in favor of object element)
 * The  element does not allow: applet, font, and basefont (elements not defined in strict DTD)
 * The  attribute on script element (presumably redundant with   attribute, though this is maintained for legacy reasons).
 * Frame related entities
 * element (used in place of body for frameset DTD)
 * element
 * attribute on, client-side image-map ,  ,  , and    elements
 * element (used in place of body for frameset DTD)
 * element
 * attribute on, client-side image-map ,  ,  , and    elements
 * attribute on, client-side image-map ,  ,  , and    elements
 * attribute on, client-side image-map ,  ,  , and    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:

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 links, transclusion, source tracking, fat links, and more. 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, members of the modern social software landscape such as wikis and content management systems allow surfers to edit the Web pages they visit.

Tutorials and guides

 * HTML Source: HTML Tutorials
 * HTML.net
 * HTMLQuick.com tutorials

HTML Markup Validators

 * W3C's Markup Validator
 * WDG HTML Validator
 * Off-line HTML Validator v1.0 for Windows

Standard HTML specifications

 * Refer to the HTML version history section

Other specifications

 * 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.