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|HTML (HyperText Markup Language)|
An example of HTML code with syntax highlighting and line numbers
|Uniform Type Identifier:||public.html|
|Type of format:||markup language|
|Standard(s):||W3C HTML 4.01|
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), created by less-than signs (<) 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 web 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).
 What is HTML?
HTML stands for Hypertext Markup Language.
- Hypertext is ordinary text that has been dressed up with extra features, such as formatting, images, multimedia, and links to other documents.
- 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.
 History of HTML
Tim Berners-Lee created the original HTML (and many associated protocols such as HTTP) on a NeXTcube workstation using the NeXTSTEP development environment. At the time, HTML was not a specification, but a collection of tools to solve an immediate problem: the communication and dissemination of ongoing research among Berners-Lee and a group of his colleagues. His solution later combined with the emerging international and public internet to garner worldwide attention.
Early versions of HTML were defined with loose syntactic rules, which helped its adoption by those unfamiliar with web publishing. Though today, the syntax of HTML is much more strict, allowing for more precise code. Web browsers commonly made assumptions about intent and proceeded with rendering of the page. Over time, as the use of authoring tools increased, the trend in the official standards has been to create an increasingly strict language syntax. However, browsers still continue to render pages that are far from valid HTML.
HTML is defined in formal specifications that were developed and published throughout the 1990s, inspired by Tim Berners-Lee‘s prior proposals to graft hypertext capability onto a homegrown SGML-like markup language for the Internet. The first published specification for a language called HTML was drafted by Berners-Lee with Dan Connolly, and was published in 1993 by the IETF as a formal “application” of SGML (with an SGML Document Type Definition defining the grammar). The IETF created an HTML Working Group in 1994 and published HTML 2.0 in 1995, but 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 and its issues and errors were last acknowledged by errata published in 2001.
Since the publication of HTML 4.0 in late 1997, the W3C’s HTML Working Group has increasingly — and from 2002 through 2006, exclusively — focused on the development of XHTML, an XML-based counterpart to HTML that is described on one W3C web page as HTML’s “successor”. XHTML applies the more rigorous, less ambiguous syntax requirements of XML to HTML to make it easier to process and extend, and as support for XHTML has increased in browsers and tools, it 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 2006, 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, and, as of 2006, they continue to be listed alongside XHTML as current Recommendations in the W3C’s primary publication indices.
In November 2006, the HTML Working Group published a new charter indicating its intent to resume development of HTML in a manner that unifies HTML 4 and XHTML 1, allowing for this hybrid language to manifest in both an XML format and a “classic HTML” format that is SGML-compatible but not strictly SGML-based. Among other things, it is planned that the new specification, to be released and refined throughout 2007 through 2008, will include conformance and parsing requirements, DOM APIs, and new widgets and APIs. The group also intends to publish test suites and validation tools.
 Version history of the standard
Layout engine comparison
Unicode and HTML
Web browsers comparison
- Hypertext Markup Language (First Version), published June 1993 as an Internet Engineering Task Force (IETF) working draft (not standard).
- HTML 2.0, published November 1995 as IETF 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.
- HTML 3.2, published January 14, 1997 as a W3C Recommendation.
- HTML 4.0, published December 18, 1997 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 4.01, published December 24, 1999 as a W3C Recommendation. It offers the same three flavors as HTML 4.0, and its last errata was published May 12, 2001.
- ISO/IEC 15445:2000 (“ISO HTML”, based on HTML 4.01 Strict), published May 15, 2000 as an ISO/IEC international standard.
HTML 4.01 and ISO/IEC 15445:2000 are the most recent and final versions of HTML. 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 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.
There is no official standard HTML 1.0 specification because there were multiple informal HTML standards at the time. Berners-Lee’s original version did not include an IMG element type. Work on a successor for HTML, then called “HTML+”, began in late 1993, designed originally to be “A superset of HTML…which will allow a gradual rollover from the previous format of HTML”. The first formal specification was therefore given the version number 2.0 in order to distinguish it from these unofficial “standards”. Work on HTML+ continued, but it never became a standard.
The HTML 3.0 standard was proposed by the newly formed W3C in March 1995, and provided many new capabilities 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, and 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 which had been created for the Netscape and Mosaic web browsers. Math support as proposed by HTML 3.0 finally came about years later with a different standard, MathML.
HTML 4.0 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.
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 filesystems, 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.
 HTML as a hypertext format
HTML is the basis of a comparatively weak hypertext implementation. Earlier hypertext systems had features such as typed links, transclusion and source tracking. Another feature lacking today is fat links.
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 editable web pages.
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.
 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 read 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:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
This declaration references the Strict DTD of HTML 4.01, which does not have presentational elements like <font>, 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 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.
- 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. <label>) and an end label (e.g. </label>). The element’s attributes are contained in the start label and content is located between the labels (e.g. <label>Content</label>). Some elements, such as <br>, will never have any content and do not need closing labels. Listed below are several types of markup elements used in HTML.
Structural markup describes the purpose of text. For example,
<h2>Golf</h2> establishes “Golf” as a second-level heading, which would be rendered in a browser in a manner similar to the “Markup element types” title at the start of this section. A blank line is included after the header. 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
<b>boldface</b> indicates that visual output devices should render “boldface” in bold text, but has no clear semantics for aural devices that read the text aloud for the sight-impaired. In the case of both
<i>italic</i> there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely
<strong>strong emphasis</strong> and
<em>emphasis</em> 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:
<a>Wikipedia</a>. However, the
href attribute must also be set to a valid URL so for example the HTML code,
<a href="http://en.wikipedia.org/">Wikipedia</a>, will render the word “ ” as a hyperlink. In order to view the HTML code in a website click → View → Source.
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:
title. Most also take language-related attributes:
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. The
class 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
class="notation" 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
style non-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s son-
id 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
title is used to attach subtextual explanation to an element. In most browsers this
title attribute is displayed as what is often referred to as a tooltip. The generic inline
span element can be used to demonstrate these various non-attributes.
- <span id=’anId’ class=’aClass’ style=’color:red;’ title=’HyperText Markup Language’>HTML</span>
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 “<” and “&” (when written as
&, respectively) to be interpreted as character data, rather than markup. For example, a literal “<” normally indicates the start of a label, and “&” normally indicates the start of a character entity reference or numeric character reference; writing it as “&” 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 “” 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
é or as the numeric references
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.
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 (
<em>) and the italics element (
<i>). 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
<span>), 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
<cite class="booktitle">The Grapes of Wrath</cite>. Here, the
<cite> element is used, because it most closely matches the meaning of this phrase in the text. However, the
<cite> 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
<b> (bold), and
<i> (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 <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 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 — something inherently meant for audible presentation — 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 may cause 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.
 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 often 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 intends XHTML 1.0 to be identical with HTML 4.01 except in the often stricter requirements of XML over traditional HTML. XHTML 1.0 likewise has three sub-specifications: strict, loose and frameset. The strictness of XHTML in terms of its syntax is often confused with the strictness of the strict versus the loose definitions in terms of the content rules of the specifications. The strictness of XML lies in the need to: always explicitly close elements (<h1>); and to always use quotation-marks (double ” or single ‘) to enclose attribute values. The use of implied closing labels in HTML led to confusion for both editors and parsers.
Aside from the different opening declarations for a document, the differences between HTML 4.01 and XHTML 1.0 — in each of the corresponding DTDs — is largely syntactic. Adhering to valid and well-formed XHTML 1.0 will result in a well-formed HTML 4.01 document in every way, except one. XHTML introduces a new markup in a self-closing element as short-hand for handling empty elements. The short-hand adds a slash (/) at the end of an opening label like this:
<br/>. The introduction of this short-hand, undefined in any HTML 4.01 DTD, may confuse earlier software unfamiliar with this new convention. To help with the transition, the W3C recommends also including a space character before the slash like this:
<br />. As validators and browsers adapt to this evolution in the standard, the migration from traditional to XML-based HTML should be relatively simple. The major problems occur when software is non-conforming to HTML 4.01 and its associated protocols to begin with, or erroneously implements the HTML recommendations.
To understand the subtle differences between HTML and XHTML consider the transformation of a valid and well-formed XHTML 1.0 document into a valid and well-formed HTML 4.0. To make this translation requires the following steps::
- The language code for the element should be specified with a
langrather than the XHTML
xml:langattribute HTML 4.01 instead defines its own attribute for language) whereas XHTML uses the XML defined attribute.
- Remove the XML namespace (
xmlns=URI). HTML does not require and has no facilities for namespaces.
- Change the DTD declaration from XHTML 1.0 to HTML 4.01. (see DTD section for further explanation]]).
- If present, remove the XML declaration (Typically this is:
<?xml version="1.0" encoding="utf-8"?>).
- Change the document’s mime type to text/html This may come from a
metaelement, from the HTTP header of the server or possibly from a filename extension (for example, change .xhtml to html).
- Change the XML empty label short-cut to a standard opening label (
Those are the only changes necessary to translate a document from XHTML 1.0 to HTML 4.01. The reverse operation can be much more complicated. HTML 4.01 allows the omission of many labels in a complex pattern derived by determining which labels are (in some sense) redundant for a valid document. In other words if the document is authored precisely to the associated HTML 4.01 content model, some labels need not be expressed. For example, since a paragraph cannot contain another paragraph, when an opening paragraph label is followed by another opening paragraph label, this implies the previous paragraph element is now closed. Similarly, elements such as
br have no allowed content, so HTML does not require an explicit closing label for this element. Also since HTML was the only specification targeted by user-agents (browsers and other HTML consuming software), the specification even allows the omission of opening and closing labels for
body, if the document’s head has no content. To translate from HTML to XHTML would first require the addition of any omitted closing labels (or using the closing label shortcut for empty elements like
Notice how XHTML’s requirement to always include explicit closing labels, allows the separation between the concepts of valid and well-formed. A well-formed XHTML document adheres to all the syntax requirements of XML. A valid document adheres to the content specification for XHTML. In other words a valid document only includes content, attributes and attribute values within each element in accord with the specification. If a closing label is omitted, an XHTML parser can first determine the document is not well-formed. Once the elements are all explicitly closed, the parser can address the question of whether the document is also valid. For an HTML parse these separate aspects of a document are not discernible. If a paragraph opening label (
p) is followed by a
div, is it because the document is not well-formed (the closing paragraph label is missing) or is the document invalid (a
div does not belong in a paragraph)? 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.
The W3C recommends several conventions to ensure an easy migration between HTML and XHTML (see HTML Compatibility Guidelines). Basically the W3C recommends:
- Including both
langattributes on any elements assigning language.
- Using the self-closing label only for elements specified as empty
- Make all label names and attribute names lower-case.
- Ensuring all attribute values are quoted with either single quotes (‘) or double quotes (“)
- Including an extra space in self-closing labels: for example
<br />instead of
- Including explicit close labels for elements that permit content but are left empty (for example, “
</img>“, not “
<img />” )
Note that by carefully following the W3C’s compatibility guidelines the difference between the resulting HTML 4.01 document and the XHTML 1.0 document is merely the DOCTYPE declaration, and the XML declaration preceding the document’s contents. The W3C allows the resulting XHTML 1.0 (or any XHTML 1.0) document to be delivered as either HTML or XHTML. For delivery as HTML, the document’s MIME type should be set to ‘text/html’, while, for XHTML, the document’s MIME type should be set to ‘application/xhtml+xml’. When delivered as XHTML, browsers and other user agents are expected to adhere strictly to the XML specifications in parsing, interpreting, and displaying 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 (also called loose), and frameset. The difference between strict on the one hand and loose and frameset on the other, is that the strict definition tries to adhere more tightly to a presentation-free or style-free concept of a semantic HTML. The loose standard maintains many of the various presentational elements and attributes absent in the strict definition.
The primary differences making the transitional specification loose versus the strict specification (whether XHTML 1.0 or HTML 4.01) are:
- A looser content model
- Inline elements and character strings (#PCDATA) are allowed in:
- Inline elements and character strings (#PCDATA) are allowed in:
- Presentation related elements
- underline (
- strike-through (
- underline (
- Presentation related attributes
paragraph(p), and heading (
- Additional elements in loose (transitional) specification
menulist (no substitute, though unordered list is recommended; may return in XHTML 2.0 specification)
dirlist (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)
preelement does not allow: applet, font, and basefont (elements not defined in strict DTD)
languageattribute on script element (presumably redundant with
typeattribute, though this is maintained for legacy reasons).
- Frame related entities
framesetelement (used in place of body for frameset DTD)
anchor, client-side image-map (
 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:
- Any of the various head related elements.
- At least one of either: another
frameand an optional
- At least one of either: another
 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 — 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.
- ^ Raggett, Dave (1998). Raggett on HTML 4. Addison-Wesley, chap. 2: A history of HTML. ISBN 0-201-17805-2.
- ^ Development of HTML 4.01 and XHTML 1.0 occurred in parallel throughout 1998 and 1999. In early 2000, after HTML 4.01 and XHTML 1.0 were published, the HTML Working Group’s charter shifted to concentrate on XHTML. HTML working group charter (2000–2002). World Wide Web Consortium. Retrieved on 2006–09-14. HTML working group charter (2002–2004). World Wide Web Consortium. Retrieved on 2006–09-14.
- ^ “W3C has no intention to extend HTML 4 as such. Instead, further work is focusing on a reformulation of HTML in XML, namely XHTML.” – HTML Working Group Roadmap. World Wide Web Consortium. Retrieved on 2006–09-14.
- ^ The “successor” reference is only in informal prose on the HTML Home Page, cited below. Additionally, the HTML 4.0 and 4.01 Recommendations each contain a hyperlink labelled “latest version of HTML” that, as of 2006, returns a copy of the latest edition of XHTML 1.0.
- ^ HyperText Markup Language (HTML) Home Page. World Wide Web Consortium. Retrieved on 2006–09-14. – This is the W3C’s primary index of its current publications and activity relating to HTML and XHTML. It endorses both XHTML and HTML as current technologies. It also explicitly refers to HTML 4.0, 3.2, and 2.0, but not 4.01, as “Previous Versions of HTML”.
- ^ W3C Technical Reports and Publications (index of all current W3C publications). World Wide Web Consortium. Retrieved on 2006–09-14. – This is the W3C’s primary index of all its current publications. It endorses both XHTML and HTML as current technologies.
- ^ was here 22Rescinded+Recommendation%22 Google web search using terms ‘site:w3.org HTML “Rescinded Recommendation”‘. Retrieved on 2006–09-14. – This search indicates that no publications exist on the W3C web site stating that HTML has been advanced into the W3C’s Rescinded Recommendation publication track. There is likewise an absence, in the W3C’s XHTML-related Recommendations, of references or designation of any edition of HTML as ‘obsolete’, ‘superseded’, ‘replaced’, or ‘deprecated’, which are the terms the organization usually uses to discourage the use of one of its publications.  XHTML is not considered part of nor a mentioned as superseding the updates to HTML 4.01 as published in HTML 4 Errata. World Wide Web Consortium. Retrieved on 2006–09-14.
- ^ HTML Working Group charter (2006-11-22). Retrieved on 2007–01-19.
- ^ http://www.useit.com/alertbox/20050103.html
- ^ http://www.w3.org/TR/html401/intro/sgmltut.html#h-3.2.2
- ^ http://www.w3.org/TR/xhtml1/diffs.html#h-4.4
- ^ http://www.cs.tut.fi/~jkorpela/qattr.html
 See also
- Alt attribute
- Character encodings in HTML
- Cascading Style Sheets
- Dynamic HTML
- The HTML Sourcebook: The Complete Guide to HTML (historical reference from 1995)
- HTML editor
- HTML element
- HTML scripting
- Tim Berners-Lee
- Unicode and HTML
- Web colors
- List of document markup languages
- Comparison of document markup languages
- Comparison of layout engines (HTML)
- Wikipedia:WikiProject Usability/HTML