HTML explained

HTML (HyperText Markup Language)
Extension:.html, .htm
Type Code:TEXT
Uniform Type:public.html
Owner:World Wide Web Consortium & WHATWG
Genre:Markup language
Extended From:SGML
Extended To:XHTML
Standard:W3C HTML 4.01
W3C HTML 3.2

HTML, an initialism of HyperText Markup Language, is the predominant markup language for Web pages. It provides a means to describe the structure of text-based information in a document—by denoting certain text as links, headings, paragraphs, lists, and so on—and to supplement that text with interactive forms, embedded images, and other objects. HTML is written in the form of tags, surrounded by angle brackets. HTML can also describe, to some degree, the appearance and semantics of a document, and can include embedded scripting language code (such as JavaScript) which can affect the behavior of Web browsers and other HTML processors.

History of HTML


In 1980, physicist Tim Berners-Lee, who was an independent contractor at CERN, proposed and prototyped ENQUIRE, a system for CERN researchers to use and share documents. In 1989, Berners-Lee and CERN data systems engineer Robert Cailliau each submitted separate proposals for an Internet-based hypertext system providing similar functionality. The following year, they collaborated on a joint proposal, the WorldWideWeb (W3) project,[1] which was accepted by CERN.

First specifications

The first publicly available description of HTML was a document called HTML Tags, first mentioned on the Internet by Berners-Lee in late 1991.[2] It describes 22 elements comprising the initial, relatively simple design of HTML. Thirteen of these elements still exist in HTML 4.[3]

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.[4] 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.[5] 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.[6]

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.[5] Published as Request for Comments 1866, HTML 2.0 included ideas from the HTML and HTML+ drafts.[7] There was no "HTML 1.0"; the 2.0 designation was intended to distinguish the new edition from previous drafts.[8]

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).[9] 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.

Version history of the standard

HTML version timeline

November 1995: HTML 2.0 was published as IETF RFC 1866. Supplemental RFCs added capabilities:

In June 2000, all of these were declared obsolete/historic by RFC 2854.

January 1997: HTML 3.2[10] 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 1997.[11]
  • 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.[9] The ability to include mathematical formulas in HTML wasn't standardized until years later in MathML.
    December 1997: HTML 4.0[12] was published as a W3C Recommendation. It offers three "flavors":

    Initially code-named "Cougar",[13] 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.

    April 1998: HTML 4.0[14] was reissued with minor edits without incrementing the version number.
    December 1999: HTML 4.01[15] was published as a W3C Recommendation. It offers the same three flavors as HTML 4.0, and its last errata were published May 12, 2001.
    May 2000: [16] ("ISO HTML", based on HTML 4.01 Strict) was published as an ISO/IEC international standard.

    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.

    October 1991: HTML Tags, an informal CERN document listing twelve HTML tags, was first mentioned in public. November 1992.
    July 1993: Hypertext Markup Language[17] was published by the IETF as an Internet-Draft (a rough proposal for a standard). It expired in January 1994.
    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[18] 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.[19] A demonstration appeared in W3C's own Arena browser. HTML 3.0 did not succeed for several reasons. The pace of browser development, as well as the number of interested parties, had outstripped the resources of the IETF.[9] Netscape continued to introduce HTML elements that specified the visual appearance of documents,[20] contrary to the goals of the newly-formed W3C, which sought to limit HTML to describing logical structure.[21] Microsoft, a newcomer at the time, played to all sides by creating its own tags, implementing Netscape's elements for compatibility, and supporting W3C features such as Cascading Style Sheets.[9]
    January 2008: HTML5[22] was published as a Working Draft by the W3C.

    XHTML versions

    See 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:

    HTML markup

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

    The Hello world program, a common computer program employed for comparing programming languages, scripting languages, and markup languages is made of 9 lines of code in HTML, albeit line breaks and the tag, or the document type declaration, are optional:


    <!DOCTYPE html>
        <title>Hello HTML</title>
        <p>Hello World!!</p>

    This Document Type Declaration is in HTML5 format.

    If the tag is not included, the layout mode defaults to "quirks mode."


    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 a HTML document to be considered valid. An element usually has a start tag (e.g. <element-name>) and an end tag (e.g. </element-name>). The element's attributes are contained in the start tag and content is located between the tags (e.g. <element-name&nbsp;attribute="value">Content</element-name>). Some elements, such as <nowiki><br></nowiki>, do not have any content and must not have a closing tag. Listed below are several types of markup elements used in HTML.

    Structural markup describes the purpose of text. For example, <nowiki><h2>Golf</h2></nowiki> establishes "Golf" as a second-level heading, which would be rendered in a browser in a manner similar to the "HTML markup" title at the start of this section. Structural markup does not denote any specific rendering, but most Web browsers have standardized on how elements should be formatted. Text may be further styled with Cascading Style Sheets (CSS).

    Presentational markup describes the appearance of the text, regardless of its function. For example <nowiki><b>boldface</b></nowiki> 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 <nowiki><b>bold</b></nowiki> and <nowiki><i>italic</i></nowiki>, there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely <nowiki><strong>strong emphasis</strong></nowiki> and <nowiki><em>emphasis</em></nowiki> 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: <nowiki><a>Wikipedia</a></nowiki>. However, the href attribute must also be set to a valid URL so for example the HTML code, <nowiki><a&nbsp;href="">Wikipedia</a></nowiki>, will render the word "

    Wikipedia" as a hyperlink.To link on an image, the anchor tag use the following syntax: <a href="url"><img src="image.gif" alt="alternative text" width="50" height="50"></a>


    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).[27] [28] Leaving attribute values unquoted is considered unsafe.[29] 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[30] (like the ismap attribute for the img element[31]).

    Most elements can take any of several common attributes:

    The generic inline element span can be used to demonstrate these various attributes:


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

    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.

    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 &lt; and &amp; (when written as &amp;lt; and &amp;amp;, respectively) to be interpreted as character data, rather than markup. For example, a literal &lt; normally indicates the start of a tag, and &amp; normally indicates the start of a character entity reference or numeric character reference; writing it as &amp;amp; or &amp;#x26; 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;#x22; or &amp;#34; when it appears within the attribute value itself. The single-quote character ('), when used to quote an attribute value, must also be escaped as &amp;#x27; or &amp;#39; (should NOT be escaped as &amp;apos; 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 &amp;eacute; or as the numeric references &amp;#233; or &amp;#xE9;. 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.

    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.

    The 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 selecting the rendering mode—particularly to avoid the quirks mode.

    The original purpose of the doctype is 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. HTML5 validation is not DTD-based, so in HTML5 the doctype does not refer to a DTD.

    An example of an HTML 4 doctype:

    <nowiki><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" ""></nowiki>

    This declaration references the Strict DTD of HTML 4.01, which does not have presentational elements like <nowiki><font></nowiki>, 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.

    Semantic HTML

    There is no official specification called "Semantic HTML". Semantic HTML refers to a practice to create documents with HTML that contain only the author's intended meaning, without any reference to how this meaning is presented. It is part of the idea of separation of presentation and content. For example, the emphasis element (&lt;em&gt;) and the italics element (&lt;i&gt;) are functionally identical at default, but have two different meanings. The meaning behind the markup is important in semantic HTML.

    CSS is typically used to create presentational effects for semantic HTML. With CSS, a designer can create the content with the appropriate, semantic markup and then use CSS to alter the appearance of their markup. The idea is that strictly presentational elements (such as italics or bold lettering) should not be used in the code and that any use of presentational elements should have some meaning attached to it, such as a citation.

    The benefit of semantic HTML is that presentation of the text will be consistently applied, so long as the proper markup is consistently applied. Moreover, it allows for easy change of presentation, by simply editing the style sheets as well as being able to transfer the text from one site to another.

    The main disadvantage is that HTML does not contain enough markup tags to describe every single conceivable description or meaning. As such, people will typically use the division (&lt;div&gt;) tag along with a set of pre-defined classes or IDs to properly mark up text for their intended meaning. If the designer has a glut of sections or meanings that don't fit well with HTML's markup, they may be forced to use a lot of division (&lt;div&gt;) tags, which could easily obfuscate the code.

    Delivery of HTML

    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.


    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 metadata usually includes the MIME type (e.g. text/html or application/xhtml+xml) and the character encoding (see Character encodings 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.[32] The current XHTML 1.1 Working Draft also states that XHTML 1.1 documents should[33] be labeled with either MIME type. [34]

    HTML e-mail

    See 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.

    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. See 8.3 filename.

    Current flavors of HTML

    Since its inception, HTML and its associated protocols gained acceptance relatively quickly. However, no clear standards existed in the early years of the language. Though its creators originally conceived of HTML as a semantic language devoid of presentation details, practical uses pushed many presentational elements and attributes into the language, driven largely by the various browser vendors. The latest standards surrounding HTML reflect efforts to overcome the sometimes chaotic development of the language and to create a rational foundation for building both meaningful and well-presented documents. To return HTML to its role as a semantic language, the 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 flavors 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.

    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).[35]

    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: &lt;br/&gt;. The introduction of this shorthand, which is not used in the SGML declaration for HTML 4.01, may confuse earlier software unfamiliar with this new convention.

    To understand the subtle differences between HTML and XHTML, consider the transformation of a valid and well-formed XHTML 1.0 document that adheres to Appendix C (see below) into a valid HTML 4.01 document. To make this translation requires the following steps:

    1. 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.
    2. Remove the XML namespace (xmlns=URI). HTML has no facilities for namespaces.
    3. Change the document type declaration from XHTML 1.0 to HTML 4.01. (see DTD section for further explanation).
    4. If present, remove the XML declaration. (Typically this is: <?xml version="1.0" encoding="utf-8"?>).
    5. 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.
    6. Change the XML empty-element syntax to an HTML style empty element (&lt;br/&gt; to &lt;br&gt;).

    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:

    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.

    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 are the same in HTML 4 and XHTML 1.0) are:

    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:


      <!-- other head elements -->
     <!-- frameset replaces body -->
      <!-- frame definitions -->
      <!--, ... -->
      <!-- optional: alternate page body for frames-incompatible user agents -->

    Summary of flavours

    As this list demonstrates, the loose flavours 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.

    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.[36] 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.

    See also

    External links

    HTML Reference

    HTML Tutorials

    HTML Markup Validators

    Standard HTML specifications

    Other specifications

    Notes and References

    1. Tim Berners-Lee, "Information Management: A Proposal." CERN (March 1989, May 1990).
    2. Web site: First mention of HTML Tags on the www-talk mailing list. World Wide Web Consortium. 1991-10-29. 2007-04-08.
    3. Web site: Index of elements in HTML 4. World Wide Web Consortium. 1999-12-24. 2007-04-08.
    4. Web site: Re: SGML/HTML docs, X Browser (archived www-talk mailing list post). Tim Berners-Lee. 1991-12-09. 2007-06-16. SGML is very general. HTML is a specific application of the SGML basic syntax applied to hypertext documents with simple structure..
    5. Book: Raymond, Eric. The Art of Unix Programming]]. IETF and the RFC Standards Process. 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..
    6. Web site: [ 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..
    7. Web site: RFC 1866: Hypertext Markup Language - 2.0 - Acknowledgments. Internet Engineering Task Force. 2005-09-22. 2007-06-16. 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..
    8. Web site: RFC 1866: Hypertext Markup Language - 2.0 - Introduction. Internet Engineering Task Force. 2005-09-22. 2007-06-16. 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..
    9. Book: Raggett, Dave. Raggett on HTML 4. 1998. 2007-07-09.
    10. Web site: HTML 3.2 Reference Specification. World Wide Web Consortium. 14-Jan-1997. 2008-11-16.
    11. Web site: IETF HTML WG. 2007-06-16. NOTE: This working group is closed.
    12. Web site: HTML 4.0 Specification. World Wide Web Consortium. 18-Dec-1997. 2008-11-16.
    13. Web site: Introduction to Wilbur. Arnoud Engelfriet. Web Design Group. 2007-06-16.
    14. Web site: HTML 4.0 Specification. World Wide Web Consortium. 24-Apr-1998. 2008-11-16.
    15. Web site: HTML 4.01 Specification. World Wide Web Consortium. 24 December 1999. 2008-11-16.
    17. Hypertext Markup Language: A Representation of Textual Information and MetaInformation for Retrieval and Interchange
    18. Web site: HTML 3.0 Draft (Expired!) Materials. World Wide Web Consortium. 1995/12/21. 2008-11-16.
    19. Web site: HyperText Markup Language Specification Version 3.0. 2007-06-16.
    20. Web site: Extensions to HTML 3.0. 2006-11-13. Netscape. Netscape remains committed to supporting HTML 3.0. To that end, we've gone ahead and implemented several of the more stable proposals, in expectation that they will be approved. …In addition, we've also added several new areas of HTML functionality to Netscape Navigator that are not currently in the HTML 3.0 specification. We think they belong there, and as part of the standards process, we are proposing them for inclusion..
    21. Web site: Press Release: W3C Publishes Public Draft of CSS2. World Wide Web Consortium. 4 November, 1997 SOPHIA-ANTIPOLIS, FRANCE. 2008-11-16.
    22. Web site: HTML 5. World Wide Web Consortium. 10 June 2008. 2008-11-16.
    23. Web site: XHTML 1.0: The Extensible HyperText Markup Language (Second Edition). World Wide Web Consortium. 26 January 2000. 2008-11-16.
    24. Web site: XHTML 1.1 - Module-based XHTML - Second Edition. World Wide Web Consortium. 16 February 2007. 2008-11-16.
    25. Web site: XHTM 2.0. World Wide Web Consortium. 26 July 2006. 2008-11-16.
    26. Web site: HTML5. World Wide Web Consortium. 24 October 2008. 2008-11-16.
    27. Web site: On SGML and HTML. World Wide Web Consortium. 2008-11-16.
    28. Web site: XHTML 1.0 - Differences with HTML 4. World Wide Web Consortium. 2008-11-16.
    29. Web site: Jukka. Korpela. Why attribute values should always be quoted in HTML. 1998-07-06. 2008-11-16.
    30. Web site: Tags used in HTML. World Wide Web Consortium. 1992-11-03. 2008-11-16.
    31. Web site: Objects, Images, and Applets in HTML documents. World Wide Web Consortium. 1999-12-24. 2008-11-16.
    32. Web site: XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition). World Wide Web Consortium. 2000, revised 2002. 7 Dec 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]..
    33. Web site: RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. Harvard University. 1997. 7 Dec 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..
    34. Web site: XHTML™ 1.1 - Module-based XHTML - Second Edition. World Wide Web Consortium. 2007. 7 Dec 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]..
    35. See e.g., XHTML#Relationship to HTML
    36. Web site: Reviving Advanced Hypertext. Jakob Nielsen. 2005-01-03. 2007-06-16.