HTML,htm HyperText Markup Language, What is HTML

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.

OHTML is also often used to refer to content in specific languages, such as a 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).

By convention, HTML format data files use a file extension .html or .htm.

History of HTML

Origins

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][3] It describes 22 elements comprising the initial, relatively simple design of HTML. Thirteen of these elements still exist in HTML 4.

Berners-Lee considered HTML to be, at the time, an application of SGML, but it was not formally defined as such until the mid-1993 publication, by the IETF, of the first proposal for an HTML specification: Berners-Lee and Dan Connolly's "Hypertext Markup Language (HTML)" Internet-Draft, which included an SGML Document Type Definition to define the grammar.[5] 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.[6] Similarly, Dave Raggett's competing Internet-Draft, "HTML+ (Hypertext Markup Format)", from late 1993, suggested standardizing already-implemented features like tables and fill-out forms.

After the HTML and HTML+ drafts expired in early 1994, the IETF created an HTML Working Group, which in 1995 completed "HTML 2.0", the first HTML specification intended to be treated as a standard against which future implementations should be based.[6] Published as Request for Comments 1996, HTML 2.0 included ideas from the HTML and HTML+ drafts.[8] There was no "HTML 1.0"; the 2.0 designation was intended to distinguish the new edition from previous drafts.

Further development under the auspices of the IETF was stalled by competing interests. Since 1996, the HTML specifications have been maintained, with input from commercial software vendors, by the World Wide Web Consortium (W3C).[7] 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 versions

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

November, 1995: HTML 2.0 published as IETF Request for Comments:
* RFC 1866,
* supplemented by RFC 1867 (form-based file upload) that same month,
* RFC 1942 (tables) in May 1996,
* RFC 1980 (client-side image maps) in August 1996, and
* RFC 2070 (internationalization) in January 1997;
Ultimately, all were declared obsolete/historic by RFC 2854 in June 2000.

An HTML 3.0 standard was proposed to the IETF by Dave Raggett and the newly formed W3C in April 1995. It proposed many of the capabilities that were in Raggett's HTML+ proposal, such as support for tables, text flow around figures, and the display of complex mathematical elements.[10] Even though it was designed to be compatible with HTML 2.0, it was too complex at the time to be implemented. Browser vendors opted to support only parts of the proposal, but implemented other markup constructs that they wanted to be incorporated into the standard.[11] When the draft expired in September 1995, work in this direction was discontinued due to lack of browser support. HTML 3.1 was never officially proposed, and the next standard proposal was HTML 3.2 (code-named "Wilbur"), which dropped the majority of the new features in HTML 3.0 and instead adopted many browser-specific element types and attributes that had been created for the Netscape and Mosaic Web browsers.

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

HTML 3.2 was never submitted to the IETF, whose HTML Working Group closed in September 1996;[13] it was instead published as one of the W3C's first "Recommendations" in early 1997. Mathematical support as proposed by HTML 3.0 finally came about years later with a different standard, MathML.

December 18, 1997: HTML 4.0, published as a W3C Recommendation. It offers three "flavors":

* Strict, in which deprecated elements are forbidden,
* Transitional, in which deprecated elements are allowed,
* Frameset, in which mostly only frame related elements are allowed;

HTML 4.0 (initially code-named "Cougar")[12] 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 in 1998 without incrementing the version number and further minor revisions as HTML 4.01.

April 24, 1998: HTML 4.0 was reissued with minor edits without incrementing the version number.

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

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

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

January 22, 2008: HTML 5, published as a Working Draft by W3C.

XHTML versions

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

* XHTML 1.0, published January 26, 2000 as a W3C Recommendation, later revised and republished August 1, 2002. It offers the same three flavors as HTML 4.0 and 4.01, reformulated in XML, with minor restrictions.
* XHTML 1.1, published May 31, 2001 as a W3C Recommendation. It is based on XHTML 1.0 Strict, but includes minor changes, can be customized, and is reformulated using modules from Modularization of XHTML, which was published April 10, 2001 as a W3C Recommendation.
* XHTML 2.0 is still a W3C Working Draft. XHTML 2.0 is incompatible with XHTML 1.x and, therefore, would be more accurate to characterize as an XHTML-inspired new language than an update to XHTML 1.x.
* XHTML 5, which is an update to XHTML 1.x, is being defined alongside HTML 5 in the HTML 5 draft.

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. HTML Hello World:

<html>
<head>
<title>Hello HTML</title>
</head>
<body>
Hello World!
</body>
</html>

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. 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 attribute="value">Content</element-name>). Some elements, such as , 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, <h2>Golf</h2> 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 boldface 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 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. 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://roshansubeid.com/">Roshan Subedi</a>, 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" /></a>

Attributes

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).[14][15] Leaving attribute values unquoted is considered unsafe.[16] 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[17] (like the ismap attribute for the img element[18]).

Most elements can take any of several common attributes:

* 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 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 document might use the designation class="notation" to indicate that all elements with this class value are subordinate to the main text of the document. Such elements might be gathered together and presented as footnotes on a page instead of appearing in the place where they occur in the HTML source.

* 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 attribute is used to attach subtextual explanation to an element. In most browsers this attribute is displayed as what is often referred to as a tooltip.

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

HTML

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

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 < 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 & 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 " or " when it appears within the attribute value itself. The single-quote character ('), when used to quote an attribute value, must also be escaped as ' or ' (should NOT be escaped as ' 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 é or as the numeric references é or 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.

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

In order to enable Document Type Definition (DTD)-based validation with SGML tools and in order to avoid the quirks mode in browsers, HTML documents can start with a Document Type Declaration (informally, a "DOCTYPE"). 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 necessarily read the DTD, however. The most popular graphical browsers use DOCTYPE declarations (or the lack thereof) and other data at the beginning of sources to determine which rendering mode to use.

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 , 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.

Source: http://en.wikipedia.org/wiki/HTML