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An excerpt of HTML code with syntax highlighting and line numbers.

In computing, HyperText Markup Language (HTML) is a markup language designed for the creation of web pages with hypertext and other information to be displayed in a web browser. HTML is used to structure information — denoting certain text as headings, paragraphs, lists and so on — and can be used to describe, to some degree, the appearance and semantics of a document. HTML's grammar structure is the HTML DTD that was created using SGML syntax.

Originally defined by Tim Berners-Lee and further developed by the IETF, HTML is now an international standard (ISO/IEC 15445:2000). Later HTML specifications are maintained by the World Wide Web Consortium (W3C).

Early versions of HTML were defined with looser syntactic rules which helped its adoption by those unfamiliar with web publishing. Web browsers commonly made assumptions about intent and proceeded with rendering of the page. Over time, 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.

XHTML, which applies the stricter rules of XML to HTML to make it easier to process and maintain, is the W3C's successor to HTML. As such, many consider XHTML to be the "current version" of HTML, but it is a separate, parallel standard; the W3C continues to recommend the use of either XHTML 1.1, XHTML 1.0, or HTML 4.01 for web publishing.

Contents

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 specify which version of the HTML standard they conform to, all HTML documents should start with a Document Type Declaration (informally, a "DOCTYPE"), which makes reference to a Document Type Definition (DTD). The DTD contains machine readable code specifying the permitted and prohibited content for a document conforming to such a DTD. For example:

"http://www.w3.org/TR/html4/strict.dtd"> This declaration asserts that the document conforms to the Strict DTD of HTML 4.01, which is purely structural, leaving formatting to Cascading Style Sheets. In some cases, the presence or absence of an appropriate DTD may influence how a web browser will display the page.

In addition to the Strict DTD, 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

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. Listed below are several types of markup elements used in HTML (see HTML elements for more detailed description of elements).

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 "Markup element types" 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 has no clear semantics for aural devices that read the text aloud for the sight-impaired. Presentational markup is largely deprecated, and presentation should be controlled by using CSS. In the case of both bold and italic there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely strong emphasis and emphasis respectively. It is easier to see how an aural user agent should interpret the latter two elements. 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 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 ["Wikipedia"] as a hyperlink.

Attributes

In the latest specification of HTML most elements take any of several common attributes: id, class, style and title. Most also take language related attributes: lang and dir.

The id attribute provides a document-wide unique identifier for an element. This can be used by stylesheets to provide presentational properties, by browsers to locate 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 attribute to add presentational properties to a particular element. It is considered better practice to use an element’s id attribute 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 attributes.

:HTML (pointing the cursor at the acronym should display the full 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 treated as equivalent and are rendered identically.

The ability to write characters in this way allows for "<" and "&" (when written as &lt; and &amp;, respectively) to be interpreted as character data, rather than markup. It 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 &eacute; or as the numeric references &#233; or &#xE9;. 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 authors 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, to emphasize something is different than to list the title of a book, for example, which may also be displayed in italics. In a purely semantic HTML a book title would use a separate element than emphasized text uses: 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 it's 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, quoted, and lists, instead of using markup which is written for visual purposes only, like <font>, <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. For example, when enter a number should it be written out in words: two? Or should it be displayed as a numeral: 2? A semantic markup might simply enter something like 2 and leave presentation details to the stylesheet designers. Similarly an author might wonder where to break out quotations into a separate indented block of text. With a purely semantic HTML, such details would again 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 (though 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 are computer files, much like any other. So they can be delivered in the same ways other computer files are delivered. 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

Some graphical e-mail clients allow the use of a subset of HTML (often ill-defined) as a pure display language. Many of these clients include a GUI HTML editor for composing e-mails and a rendering engine for displaying them once received. Use of HTML in e-mail is quite controversial due to a variety of issues. The main benefit is the ability to decorate an e-mail with presentational attributes (bold headings etc). However, there are a number of disadvantages, which include:

  • the recipient may not have an e-mail client that can display HTML
  • the e-mail has larger size because lots of formatting will be much larger than the plain text equivalent. This issue is made slightly worse by the fact that, for compatibility, most clients send a plain-text version as well.
  • overuse of formatting (there was at one stage a craze for making letterheads using HTML and sending them as part of every e-mail)
  • potential security issues of deluding the recipient to accept an e-mail as being from an authoritative source (such as a bank) when this is not the case; this is related to phishing scams.
  • potential security issues of simply rendering a complex format like HTML, particularly if the object, embed, iframe or script tags are included as tags to be parsed.
  • potential privacy issues when embedding external content such as images, which can alert a third party that an e-mail has been read (some e-mail clients do not load external images by default for this reason).
For these reasons many mailing lists deliberately block HTML e-mail either stripping out the HTML part to just leave the plain text part or rejecting the entire message.

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 W3 has developed style languages such as CSS and XSL to shoulder the burden of presentation. In conjunction the HTML specification has slowly reigned 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.

SGML 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 specification is often called XHTML to clearly distinguish it from the SGML based definition. However, the root element name continues to be HTML even in the XHTML specified HTML. The W3 intends XHTML 1.01 to be identical with HTML 4.01 except in the often stricter requirements of XML over SGML. XHTML 1.01 likewise has three sub-specifications: strict, loose and frameset. The strictness of XML base XHTML in terms of semantics 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 provide closing tags for elements (<h1>); always use quotation-marks (" or ') to enclose attribute values; and include xml declarations and doctype definitions to begin a document.

Aside from the different opening declarations for a document, the differences between HTML 4.01 and XHTML 1.01 — in each of the corresponding DTDs — is largely syntactic. Adhering to valid XHTML 1.01 will result in a valid HTML 4.01 document in every way, except one. XHTML introduces a new markup in a self-closing element as short-hand for handling elements whose content model specifies empty. The short-hand adds a slash (/) at the end of an opening tag 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 W3 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 SGML 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 XHTML and XML protocols.

The latest SGML based specification HTML 4.01 and the earliest XHTML version include three sub-specifications: 1) strict, 2)transitional (also called loose) and 3) 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.

Transitional versus Strict

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: body, blockquote, form, noscript, noframes
  • Presentation related elements
  • * underline (u)
  • * strike-through (s and strike)
  • * center
  • * font
  • * basefont
  • Presentation related attributes
  • * background and backgroundcolors attributes for body element.
  • * align attribute on div, form, paragraph (p), and heading (h1...h6) elements
  • * align, noshade, size, and width attributes on hr element
  • * align, border, vspace, and hspace attributes on img and object elements
  • * align attribute on legend and caption elements
  • * align and backgroundcolor on table element
  • * nowrap,, bgcolor, width, height on td and th elements
  • * bgcolor attribute on tr element
  • * clear attribute on br element
  • * compact attribute on dl, dir and menu elements
  • * type, compact, and start attributes on ol and ul elements
  • * type and value attributes on li element
  • * width attribute on pre element
  • Additional elements in loose (transitional) specification
  • * men list (no substitute, though unordered list is recommended; may return in XHTML 2.0 specification)
  • * dir list (no substitute, though unordered list is recommended)
  • * isindex (element requires server-side support and is typically added to documents server-side)
  • * applet (deprecated in favor of object element)
  • The pre element does not allow: applet, font, and basefont (elements not defined in strict DTD)
  • The language attribute on script element (presumably redundant with type attribute, though this is maintained for legacy reasons).
  • Frame related entities
  • * frameset element (used in place of body for frameset DTD)
  • * frame element
  • * iframe
  • * noframes
  • * target attribute on anchor, client-side image-map (imagemap), link, form, and base elements

Frameset versus Transitional

In addition to the above transitional differences, the frameset specifications (whether XHTML 1.01 or HTML 4.01) specifies a different content model:

<html>
:<head>
::Any of the various head related elements.
:</head>
:<frameset>
::At least one of either: another frameset or a frame and an optional noframes element.
:</frameset>
</html>

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 W3 is modularizing the entire specification and opening it up to independent extensions. The primary achievement in the move from XHTML 1.01 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 module)s. 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 reigned in all the various HTML implementations into a single clear written specification based on SGML. XHTML 1.01, 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.

History of HTML

Tim Berners-Lee, created the original HTML (and many of the associated protocols such as HTTP) on a NeXTStep workstation using the NeXTStep development environment. THen, HTML was not a specification, but a collection of tools to solve an immediate problem: the communication and dissemination of ongoing research among Tim Berners-Lee and a group of his colleagues. His solution later combined with the emerging international and public internet to garner worldwide attention. Below is a capsule history of the various work to develop HTML into the specifications it is today.

Version history of the standard

  • [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. HTML's successor, 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. However, some people consider the initial edition provided by Tim Berners-Lee to be the definitive HTML 1.0. That 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, however, older operating systems, such as DOS, limit file extensions to three letters, so a .htm extension is also used. Although perhaps less common now, the shorter form is still widely supported by current software.

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 now, 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.

See also: [Jacob Nielsen on advanced hypertext for the World Wide Web].

See also

External links

Wikibooks has a manual, textbook or guide to this subject:
[[wikibooks:|]]

W3C Specifications

Selected tutorials and guides

Validators

[[bat-smg:HTML]]

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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