|HTML (HyperText Markup Language)|
|Owner:||World Wide Web Consortium & WHATWG|
|Standard:||W3C HTML 4.01|
W3C HTML 3.2
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, which was accepted by CERN.
The first publicly available description of HTML was a document called HTML Tags, first mentioned on the Internet by Berners-Lee in late 1991. 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. 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. 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. Published as Request for Comments 1866, HTML 2.0 included ideas from the HTML and HTML+ drafts. 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). 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.
In June 2000, all of these were declared obsolete/historic by RFC 2854.
Initially code-named "Cougar", 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.
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.
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> <html> <head> <title>Hello HTML</title> </head> <body> <p>Hello World!!</p> </body> </html>
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 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><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 href="http://en.wikipedia.org/">Wikipedia</a></nowiki>, will render the word "
<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).  Leaving attribute values unquoted is considered unsafe. 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 (like the
ismap attribute for the
Most elements can take any of several common attributes:
idattribute 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.
classattribute 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.
stylenon-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s son-
idpage and select the element with a stylesheet, though sometimes this can be too cumbersome for a simple ad hoc application of styled properties.
titleattribute 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:
<span id="anId" class="aClass" style="color:blue;" title="Hypertext Markup Language">HTML</span>
This example displays asHTML; 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
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
& (when written as
&amp;, 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
& 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
&#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
&#39; (should NOT be escaped as
&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
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
&#xE9;. The characters comprising those references (that is, 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 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.
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" "http://www.w3.org/TR/html4/strict.dtd"></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
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.
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 (
<em>) and the italics element (
<i>) 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 (
<div>) 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 (
<div>) tags, which could easily obfuscate the code.
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.
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. The current XHTML 1.1 Working Draft also states that XHTML 1.1 documents should be labeled with either MIME type. 
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.
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.
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.
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).
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:
<br/>. 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:
langattribute rather than the XHTML
xml:langattribute. XHTML uses XML's built in language-defining functionality attribute.
xmlns=URI). HTML has no facilities for namespaces.
<?xml version="1.0" encoding="utf-8"?>).
text/html. For both HTML and XHTML, this comes from the HTTP
Content-Typeheader sent by the server.
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:
langattributes on any elements assigning language.
<br />instead of
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/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.
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:
form, paragraph (
p), and heading (
menulist (no substitute, though unordered list is recommended; may return in XHTML 2.0 specification)
dirlist (no substitute, though unordered list is recommended)
isindex(element requires server-side support and is typically added to documents server-side)
applet(deprecated in favor of object element)
languageattribute on script element (presumably redundant with
typeattribute, though this is maintained for legacy reasons).
framesetelement (used in place of body for frameset DTD)
anchor, client-side image-map (
In addition to the above transitional differences, the frameset specifications (whether XHTML 1.0 or HTML 4.01) specifies a different content model:
<html> <head> <title></title> <!-- other head elements --> </head> <!-- frameset replaces body --> <frameset> <!-- frame definitions --> <frame></frame> <!--, ... --> <!-- optional: alternate page body for frames-incompatible user agents --> <noframes> <body></body> </noframes> </frameset> </html>
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.
HTML lacks some of the features found in earlier hypertext systems, such as typed links, source tracking, fat links, and more. 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.