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In computing, HyperText Markup Language (HTML) is a 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 can also describe, to some degree, the appearance and semantics of a document, and can provide additional cues, such as embedded scripting language code, that 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).
History of HTML Tim Berners-Lee created the original HTML (and many of the 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. 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. 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 since 2002, 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".• Version history of the 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: 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 and filesystems, such as the DOS versions from the 80's and early 90's and FAT, 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. 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: 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 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 tag (eg. , will never have any content and do not need closing tags. Listed below are several types of markup elements used in HTML. Structural markup describes the purpose of text. For example, establishes "Golf" as a second-level heading, which would be rendered in a browser in a manner similar to the "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, 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 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,
will render the word "Wikipedia" as a hyperlink. Attributes 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 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: 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.
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 < and &, respectively) to be interpreted as character data, rather than markup. For example, a literal "<" normally indicates the start of a tag, and "&" normally indicates the start of a character entity reference or numeric character reference; writing it as "&" or "&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 & or &. 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 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 its own element and class specified such as The Grapes of Wrath. 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 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 A third advantage is device independence and re purposing 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. Re purposing 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, 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 vs. 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 ( ); and to always use quotation-marks (double " or single ') to enclose attribute values. The use of implied closing tags in HTML led to confusion for both editors and parsers. | |||||||||||||||||||||||||||||||
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