Development

In 1980, physicist Tim Berners-Lee, a contractor at CERN, proposed and prototyped ENQUIRE, a system for CERN researchers to use and share documents. In 1989, Berners-Lee wrote a memo proposing an Internet-based hypertext system.^[3] Berners-Lee specified HTML and wrote the browser and server software in late 1990. That year, Berners-Lee and CERN data systems engineer Robert Cailliau collaborated on a joint request for funding, but the project was not formally adopted by CERN. In his personal notes^[4] from 1990 he listed^[5] "some of the many areas in which hypertext is used" and put an encyclopedia first.

The first publicly available description of HTML was a document called "HTML Tags", first mentioned on the Internet by Tim Berners-Lee in late 1991.^[6][7] It describes 18 elements comprising the initial, relatively simple design of HTML. Except for the hyperlink tag, these were strongly influenced by SGMLguid, an in-house Standard Generalized Markup Language (SGML)-based documentation format at CERN. Eleven of these elements still exist in HTML 4.^[8]

HTML is a markup language that web browsers use to interpret and compose text, images, and other material into visual or audible web pages. Default characteristics for every item of HTML markup are defined in the browser, and these characteristics can be altered or enhanced by the web page designer's additional use of CSS. Many of the text elements are found in the 1988 ISO technical report TR 9537 Techniques for using SGML, which in turn covers the features of early text formatting languages such as that used by the RUNOFF command developed in the early 1960s for the CTSS (Compatible Time-Sharing System) operating system: these formatting commands were derived from the commands used by typesetters to manually format documents. However, the SGML concept of generalized markup is based on elements (nested annotated ranges with attributes) rather than merely print effects, with also the separation of structure and markup; HTML has been progressively moved in this direction with CSS.

Berners-Lee considered HTML to be an application of SGML. It was formally defined as such by the Internet Engineering Task Force (IETF) with the mid-1993 publication of the first proposal for an HTML specification, the "Hypertext Markup Language (HTML)" Internet Draft by Berners-Lee and Dan Connolly, which included an SGML Document type definition to define the grammar.^[9][10] 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.^[11]

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.^[12]

Further development under the auspices of the IETF was stalled by competing interests. Since 1996,^[update] the HTML specifications have been maintained, with input from commercial software vendors, by the World Wide Web Consortium (W3C).^[13] However, in 2000, HTML also became an international standard (ISO/IEC 15445:2000). HTML 4.01 was published in late 1999, with further errata published through 2001. In 2004, development began on HTML5 in the Web Hypertext Application Technology Working Group (WHATWG), which became a joint deliverable with the W3C in 2008, and completed and standardized on 28 October 2014.^[14]

HTML versions timeline

HTML 2

November 24, 1995

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

• November 25, 1995: RFC 1867 (form-based file upload)
• May 1996: RFC 1942 (tables)
• August 1996: RFC 1980 (client-side image maps)
• January 1997: RFC 2070 (internationalization)

HTML 3

January 14, 1997

HTML 3.2^[15] 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 on September 12, 1996.^[16]

Initially code-named "Wilbur",^[17] 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.^[13] A markup for mathematical formulas similar to that in HTML was not standardized until 14 months later in MathML.

HTML 4

December 18, 1997

HTML 4.0^[18] was published as a W3C Recommendation. It offers three variations:

• Strict, in which deprecated elements are forbidden
• Transitional, in which deprecated elements are allowed
• Frameset, in which mostly only frame related elements are allowed.

Initially code-named "Cougar",^[17] 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. HTML 4 is an SGML application conforming to ISO 8879 – SGML.^[19]

April 24, 1998

HTML 4.0^[20] was reissued with minor edits without incrementing the version number.

December 24, 1999

HTML 4.01^[21] was published as a W3C Recommendation. It offers the same three variations as HTML 4.0 and its last errata were published on May 12, 2001.

May 2000

ISO/IEC 15445:2000^[22][23] ("ISO HTML", based on HTML 4.01 Strict) was published as an ISO/IEC international standard. In the ISO this standard falls in the domain of the ISO/IEC JTC1/SC34 (ISO/IEC Joint Technical Committee 1, Subcommittee 34 – Document description and processing languages).^[22]

After HTML 4.01, there was no new version of HTML for many years as development of the parallel, XML-based language XHTML occupied the W3C's HTML Working Group through the early and mid-2000s.

HTML 5

October 28, 2014

HTML5^[24] was published as a W3C Recommendation.^[25]

November 1, 2016

HTML 5.1^[26] was published as a W3C Recommendation.^[27][28]

December 14, 2017

HTML 5.2^[29] was published as a W3C Recommendation.^[30][31]

HTML draft version timeline

October 1991

HTML Tags,^[6] an informal CERN document listing 18 HTML tags, was first mentioned in public.

June 1992

First informal draft of the HTML DTD,^[32] with seven^[33][34][35] subsequent revisions (July 15, August 6, August 18, November 17, November 19, November 20, November 22)

November 1992

HTML DTD 1.1 (the first with a version number, based on RCS revisions, which start with 1.1 rather than 1.0), an informal draft^[35]

June 1993

Hypertext Markup Language^[36] was published by the IETF IIIR Working Group as an Internet Draft (a rough proposal for a standard). It was replaced by a second version^[37] one month later.

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 July 1994.^[38]

November 1994

First draft (revision 00) of HTML 2.0 published by IETF itself^[39] (called as "HTML 2.0" from revision 02^[40]), that finally led to publication of RFC 1866 in November 1995.^[41]

April 1995 (authored March 1995)

HTML 3.0^[42] was proposed as a standard to the IETF, but the proposal expired five months later (28 September 1995)^[43] 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.^[43]

W3C began development of its own Arena browser as a test bed for HTML 3 and Cascading Style Sheets,^[44][45][46] but HTML 3.0 did not succeed for several reasons. The draft was considered very large at 150 pages and the pace of browser development, as well as the number of interested parties, had outstripped the resources of the IETF.^[13] Browser vendors, including Microsoft and Netscape at the time, chose to implement different subsets of HTML 3's draft features as well as to introduce their own extensions to it.^[13] (see Browser wars). These included extensions to control stylistic aspects of documents, contrary to the "belief [of the academic engineering community] that such things as text color, background texture, font size and font face were definitely outside the scope of a language when their only intent was to specify how a document would be organized."^[13] Dave Raggett, who has been a W3C Fellow for many years, has commented for example: "To a certain extent, Microsoft built its business on the Web by extending HTML features."^[13]

Logo of HTML5

January 2008

HTML5 was published as a Working Draft by the W3C.^[47]

Although its syntax closely resembles that of SGML, HTML5 has abandoned any attempt to be an SGML application and has explicitly defined its own "html" serialization, in addition to an alternative XML-based XHTML5 serialization.^[48]

2011 HTML5 – Last Call

On 14 February 2011, the W3C extended the charter of its HTML Working Group with clear milestones for HTML5. In May 2011, the working group advanced HTML5 to "Last Call", an invitation to communities inside and outside W3C to confirm the technical soundness of the specification. The W3C developed a comprehensive test suite to achieve broad interoperability for the full specification by 2014, which was the target date for recommendation.^[49] In January 2011, the WHATWG renamed its "HTML5" living standard to "HTML". The W3C nevertheless continues its project to release HTML5.^[50]

2012 HTML5 – Candidate Recommendation

In July 2012, WHATWG and W3C decided on a degree of separation. W3C will continue the HTML5 specification work, focusing on a single definitive standard, which is considered as a "snapshot" by WHATWG. The WHATWG organization will continue its work with HTML5 as a "Living Standard". The concept of a living standard is that it is never complete and is always being updated and improved. New features can be added but functionality will not be removed.^[51]

In December 2012, W3C designated HTML5 as a Candidate Recommendation.^[52] The criterion for advancement to W3C Recommendation is "two 100% complete and fully interoperable implementations".^[53]

2014 HTML5 – Proposed Recommendation and Recommendation

In September 2014, W3C moved HTML5 to Proposed Recommendation.^[54]

On 28 October 2014, HTML5 was released as a stable W3C Recommendation,^[55] meaning the specification process is complete.^[56]

XHTML versions

XHTML is a separate language that began as a reformulation of HTML 4.01 using XML 1.0. It is no longer being developed as a separate standard.

• XHTML 1.0 was published as a W3C Recommendation on January 26, 2000,^[57] and was later revised and republished on August 1, 2002. It offers the same three variations as HTML 4.0 and 4.01, reformulated in XML, with minor restrictions.
• XHTML 1.1^[58] was published as a W3C Recommendation on May 31, 2001. It is based on XHTML 1.0 Strict, but includes minor changes, can be customized, and is reformulated using modules in the W3C recommendation "Modularization of XHTML", which was published on April 10, 2001.^[59]
• XHTML 2.0 was a working draft, work on it was abandoned in 2009 in favor of work on HTML5 and XHTML5.^[60][61][62] XHTML 2.0 was incompatible with XHTML 1.x and, therefore, would be more accurately characterized as an XHTML-inspired new language than an update to XHTML 1.x.
• An XHTML syntax, known as "XHTML5.1", is being defined alongside HTML5 in the HTML5 draft.^[63]

Transition of HTML Publication to WHATWG

On 28 May 2019, the W3C announced that WHATWG would be the sole publisher of the HTML and DOM standards.^{[64][65][66][67]} The W3C and WHATWG had been publishing competing standards since 2012. While the W3C standard was identical to the WHATWG in 2007 the standards have since progressively diverged due to different design decisions.^[68] The WHATWG "Living Standard" had been the de facto web standard for some time.^[69]

Markup

HTML markup consists of several key components, including those called tags (and their attributes), character-based data types, character references and entity references. HTML tags most commonly come in pairs like <h1> and </h1>, although some represent empty elements and so are unpaired, for example <img>. The first tag in such a pair is the start tag, and the second is the end tag (they are also called opening tags and closing tags).

Another important component is the HTML document type declaration, which triggers standards mode rendering.

The following is an example of the classic "Hello, World!" program:

<!DOCTYPE html>
<html>
  <head>
    <title>This is a title</title>
  </head>
  <body>
    <div>
        <p>Hello world!</p>
    </div>
  </body>
</html>

The text between <html> and </html> describes the web page, and the text between <body> and </body> is the visible page content. The markup text <title>This is a title</title> defines the browser page title shown on browser tabs and window titles, and the tag <div> defines a division of the page used for easy styling. Additionally, a <meta> element is used in between <head> and </head>, which can be used to define the webpage’s metadata.

The Document Type Declaration <!DOCTYPE html> is for HTML5. If a declaration is not included, various browsers will revert to "quirks mode" for rendering.^[70]

Elements

HTML documents imply a structure of nested HTML elements. These are indicated in the document by HTML tags, enclosed in angle brackets thus: <p>.^{[71][better source needed]}

In the simple, general case, the extent of an element is indicated by a pair of tags: a "start tag" <p> and "end tag" </p>. The text content of the element, if any, is placed between these tags.

Tags may also enclose further tag markup between the start and end, including a mixture of tags and text. This indicates further (nested) elements, as children of the parent element.

The start tag may also include element's attributes within the tag. These indicate other information, such as identifiers for sections within the document, identifiers used to bind style information to the presentation of the document, and for some tags such as the <img> used to embed images, the reference to the image resource in the format like this: <img src="example.com/example.jpg">

Some elements, such as the line break <br>, or <br /> do not permit any embedded content, either text or further tags. These require only a single empty tag (akin to a start tag) and do not use an end tag.

Many tags, particularly the closing end tag for the very commonly used paragraph element <p>, are optional. An HTML browser or other agent can infer the closure for the end of an element from the context and the structural rules defined by the HTML standard. These rules are complex and not widely understood by most HTML coders.

The general form of an HTML element is therefore: <tag attribute1="value1" attribute2="value2">''content''</tag>. Some HTML elements are defined as empty elements and take the form <tag attribute1="value1" attribute2="value2">. Empty elements may enclose no content, for instance, the <br> tag or the inline <img> tag. The name of an HTML element is the name used in the tags. Note that the end tag's name is preceded by a slash character, /, and that in empty elements the end tag is neither required nor allowed. If attributes are not mentioned, default values are used in each case.

Element examples

Header of the HTML document: <head>...</head>. The title is included in the head, for example:

<head>
  <title>The Title</title>
  <link rel="stylesheet" href="stylebyjimbowales.css" /> <!-- Imports Stylesheets -->
</head>

Headings

HTML headings are defined with the <h1> to <h6> tags with H1 being the highest (or most important) level and H6 the least:

<h1>Heading level 1</h1>
<h2>Heading level 2</h2>
<h3>Heading level 3</h3>
<h4>Heading level 4</h4>
<h5>Heading level 5</h5>
<h6>Heading level 6</h6>

The effects are:

Heading Level 1

Heading Level 2

Heading Level 3

Heading Level 4

Heading Level 5

Heading Level 6

Note that CSS can drastically change the rendering.

Paragraphs:

<p>Paragraph 1</p> <p>Paragraph 2</p>

Line breaks:

<br>. The difference between <br> and <p> is that <br> breaks a line without altering the semantic structure of the page, whereas <p> sections the page into paragraphs. The element <br> is an empty element in that, although it may have attributes, it can take no content and it may not have an end tag.

<p>This <br> is a paragraph <br> with <br> line breaks</p>

This is a link in HTML. To create a link the <a> tag is used. The href attribute holds the URL address of the link.

<a href="https://www.wikipedia.org/">A link to Wikipedia!</a>

Inputs:

There are many possible ways a user can give input/s like:

<input type="text" /> <!-- This is for text input -->
<input type="file" /> <!-- This is for uploading files -->
<input type="checkbox" /> <!-- This is for checkboxes -->

Comments:

<!-- This is a comment -->

Comments can help in the understanding of the markup and do not display in the webpage.

There are several types of markup elements used in HTML:

Structural markup indicates the purpose of text

For example, <h2>Golf</h2> establishes "Golf" as a second-level heading. Structural markup does not denote any specific rendering, but most web browsers have default styles for element formatting. Content may be further styled using Cascading Style Sheets (CSS).^[72]

Presentational markup indicates the appearance of the text, regardless of its purpose

For example, <b>bold text</b> indicates that visual output devices should render "boldface" in bold text, but gives little indication what devices that are unable to do this (such as aural devices that read the text aloud) should do. In the case of both <b>bold text</b> and <i>italic text</i>, there are other elements that may have equivalent visual renderings but that are more semantic in nature, such as <strong>strong text</strong> and <em>emphasized text</em> 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 using CSS for styling.

Hypertext markup makes parts of a document into links to other documents

An anchor element creates a hyperlink in the document and its href attribute sets the link's target URL. For example, the HTML markup <a href="https://www.google.com/">Wikipedia</a>, will render the word "Wikipedia" as a hyperlink. To render an image as a hyperlink, an img element is inserted as content into the a element. Like br, img is an empty element with attributes but no content or closing tag. <a href="https://example.org"><img src="image.gif" alt="descriptive text" width="50" height="50" border="0"></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).^[73][74] Leaving attribute values unquoted is considered unsafe.^[75] 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,^[6] like the ismap attribute for the img element.^[76]

There are several common attributes that may appear in many elements :

• The id attribute provides a document-wide unique identifier for an element. This is used to identify the element so that stylesheets can alter its presentational properties, and scripts may alter, animate or delete its contents or presentation. Appended to the URL of the page, it provides a globally unique identifier for the element, typically a sub-section of the page. For example, the ID "Attributes" in https://en.wikipedia.org/wiki/HTML#Attributes.
• The class attribute provides a way of classifying similar elements. This can be used for semantic or presentation purposes. For example, an HTML document might semantically use the designation <class="notation"> to indicate that all elements with this class value are subordinate to the main text of the document. In presentation, 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. Class attributes are used semantically in microformats. Multiple class values may be specified; for example <class="notation important"> puts the element into both the notation and the important classes.
• An author may use the style attribute to assign presentational properties to a particular element. It is considered better practice to use an element's id or class attributes to select the element from within a stylesheet, though sometimes this can be too cumbersome for a simple, specific, or ad hoc styling.
• The title attribute is used to attach subtextual explanation to an element. In most browsers this attribute is displayed as a tooltip.
• The lang attribute identifies the natural language of the element's contents, which may be different from that of the rest of the document. For example, in an English-language document:

  <p>Oh well, <span lang="fr">c'est la vie</span>, as they say in France.</p>
  

The abbreviation element, abbr, can be used to demonstrate some of these attributes:

<abbr id="anId" class="jargon" style="color:purple;" title="Hypertext Markup Language">HTML</abbr>

This example displays as HTML; in most browsers, pointing the cursor at the abbreviation should display the title text "Hypertext Markup Language."

Most elements take the language-related attribute dir to specify text direction, such as with "rtl" for right-to-left text in, for example, Arabic, Persian or Hebrew.^[77]

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 an element or in the value of an attribute. The double-quote character ("), when not used to quote an attribute value, must also be escaped as " or " or " when it appears within the attribute value itself. Equivalently, the single-quote character ('), when not used to quote an attribute value, must also be escaped as ' or ' (or as ' in HTML5 or XHTML documents^[78][79]) when it appears within the attribute value itself. If document authors overlook the need to escape such characters, some browsers can be very forgiving and try to use context to guess their intent. The result is still invalid markup, which makes the document less accessible to other browsers and to other user agents that may try to parse the document for search and indexing purposes for example.

Escaping also allows for characters that are not easily typed, or that are not available in the document's character encoding, to be represented within element and attribute content. For example, the acute-accented e (é), a character typically found only on Western European and South American keyboards, can be written in any HTML document as the entity reference é or as the numeric references é or é, using characters that are available on all keyboards and are supported in all character encodings. Unicode character encodings such as UTF-8 are compatible with all modern browsers and allow direct access to almost all the characters of the world's writing systems.^[80]

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.

Document type declaration

HTML documents are required to start with a Document Type Declaration (informally, a "doctype"). In browsers, the doctype helps to define the rendering mode—particularly whether to use quirks mode.

The original purpose of the doctype was to enable parsing and validation of HTML documents by SGML tools based on the Document Type Definition (DTD). The DTD to which the DOCTYPE refers contains a machine-readable grammar specifying the permitted and prohibited content for a document conforming to such a DTD. Browsers, on the other hand, do not implement HTML as an application of SGML and by consequence do not read the DTD.

HTML5 does not define a DTD; therefore, in HTML5 the doctype declaration is simpler and shorter:^[81]

<!DOCTYPE html>

An example of an HTML 4 doctype

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "https://www.w3.org/TR/html4/strict.dtd">

This declaration references the DTD for the "strict" version of HTML 4.01. SGML-based validators read the DTD in order to properly parse the document and to perform validation. In modern browsers, a valid doctype activates standards mode as opposed to quirks mode.

In addition, HTML 4.01 provides Transitional and Frameset DTDs, as explained below. Transitional type is the most inclusive, incorporating current tags as well as older or "deprecated" tags, with the Strict DTD excluding deprecated tags. Frameset has all tags necessary to make frames on a page along with the tags included in transitional type.^[82]

Semantic HTML

Semantic HTML is a way of writing HTML that emphasizes the meaning of the encoded information over its presentation (look). HTML has included semantic markup from its inception,^[83] but has also included presentational markup, such as <font>, <i> and <center> tags. There are also the semantically neutral span and div tags. Since the late 1990s, when Cascading Style Sheets were beginning to work in most browsers, web authors have been encouraged to avoid the use of presentational HTML markup with a view to the separation of presentation and content.^[84]

In a 2001 discussion of the Semantic Web, Tim Berners-Lee and others gave examples of ways in which intelligent software "agents" may one day automatically crawl the web and find, filter and correlate previously unrelated, published facts for the benefit of human users.^[85] Such agents are not commonplace even now, but some of the ideas of Web 2.0, mashups and price comparison websites may be coming close. The main difference between these web application hybrids and Berners-Lee's semantic agents lies in the fact that the current aggregation and hybridization of information is usually designed in by web developers, who already know the web locations and the API semantics of the specific data they wish to mash, compare and combine.

An important type of web agent that does crawl and read web pages automatically, without prior knowledge of what it might find, is the web crawler or search-engine spider. These software agents are dependent on the semantic clarity of web pages they find as they use various techniques and algorithms to read and index millions of web pages a day and provide web users with search facilities without which the World Wide Web's usefulness would be greatly reduced.

In order for search-engine spiders to be able to rate the significance of pieces of text they find in HTML documents, and also for those creating mashups and other hybrids as well as for more automated agents as they are developed, the semantic structures that exist in HTML need to be widely and uniformly applied to bring out the meaning of published text.^[86]

Presentational markup tags are deprecated in current HTML and XHTML recommendations. The majority of presentational features from previous versions of HTML are no longer allowed as they lead to poorer accessibility, higher cost of site maintenance, and larger document sizes.^[87]

Good semantic HTML also improves the accessibility of web documents (see also Web Content Accessibility Guidelines). For example, when a screen reader or audio browser can correctly ascertain the structure of a document, it will not waste the visually impaired user's time by reading out repeated or irrelevant information when it has been marked up correctly.

Delivery

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 email.

HTTP

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 meta data usually includes the MIME type (e.g., text/html or application/xhtml+xml) and the character encoding (see Character encoding 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; 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.^[88] XHTML 1.1 also states that XHTML 1.1 documents should^[89] be labeled with either MIME type.^[90]

HTML e-mail

Most graphical email 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 criticized by some because of compatibility issues, because it can help disguise phishing attacks, because of accessibility issues for blind or visually impaired people, 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 the limitations imposed by FAT data structure, limited file extensions to three letters.^[91]

HTML Application

An HTML Application (HTA; file extension ".hta") is a Microsoft Windows application that uses HTML and Dynamic HTML in a browser to provide the application's graphical interface. A regular HTML file is confined to the security model of the web browser's security, communicating only to web servers and manipulating only web page objects and site cookies. An HTA runs as a fully trusted application and therefore has more privileges, like creation/editing/removal of files and Windows Registry entries. Because they operate outside the browser's security model, HTAs cannot be executed via HTTP, but must be downloaded (just like an EXE file) and executed from local file system.

HTML4 variations

Since its inception, HTML and its associated protocols gained acceptance relatively quickly.^{[by whom?]} 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,^[92] 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^[93] 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 variations 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).

Like HTML 4.01, XHTML 1.0 has three sub-specifications: strict, transitional 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 and 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. A fix for this is to include a space before closing the tag, as such: <br />.^[94]

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 (<br /> to <br>).

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:

• Include both xml:lang and lang attributes on any elements assigning language.
• Use the empty-element syntax only for elements specified as empty in HTML.
• Include an extra space in empty-element tags: for example <br /> instead of <br>.
• Include explicit close tags for elements that permit content but are left empty (for example, <div></div>, not <div />).
• Omit the XML declaration.

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

HTML 4 defined three different versions of the language: Strict, Transitional (once called Loose) and Frameset. The Strict version is intended for new documents and is considered best practice, while the Transitional and Frameset versions were developed to make it easier to transition documents that conformed to older HTML specification or didn't conform to any specification to a version of HTML 4. The Transitional and Frameset versions allow for presentational markup, which is omitted in the Strict version. Instead, cascading style sheets are encouraged to improve the presentation of HTML documents. Because XHTML 1 only defines an XML syntax for the language defined by HTML 4, the same differences apply to XHTML 1 as well.

The Transitional version allows the following parts of the vocabulary, which are not included in the Strict version:

• A looser content model
  • Inline elements and plain text are allowed directly in: body, blockquote, form, noscript and noframes
• Presentation related elements
  • underline (u)(Deprecated. can confuse a visitor with a hyperlink.)
  • strike-through (s)
  • center (Deprecated. use CSS instead.)
  • font (Deprecated. use CSS instead.)
  • basefont (Deprecated. use CSS instead.)
• Presentation related attributes
  • background (Deprecated. use CSS instead.) and bgcolor (Deprecated. use CSS instead.) attributes for body (required element according to the W3C.) element.
  • align (Deprecated. use CSS instead.) attribute on div, form, paragraph (p) and heading (h1...h6) elements
  • align (Deprecated. use CSS instead.), noshade (Deprecated. use CSS instead.), size (Deprecated. use CSS instead.) and width (Deprecated. use CSS instead.) attributes on hr element
  • align (Deprecated. use CSS instead.), border, vspace and hspace attributes on img and object (caution: the object element is only supported in Internet Explorer (from the major browsers)) elements
  • align (Deprecated. use CSS instead.) attribute on legend and caption elements
  • align (Deprecated. use CSS instead.) and bgcolor (Deprecated. use CSS instead.) on table element
  • nowrap (Obsolete), bgcolor (Deprecated. use CSS instead.), width, height on td and th elements
  • bgcolor (Deprecated. use CSS instead.) attribute on tr element
  • clear (Obsolete) attribute on br element
  • compact attribute on dl, dir and menu elements
  • type (Deprecated. use CSS instead.), compact (Deprecated. use CSS instead.) and start (Deprecated. use CSS instead.) attributes on ol and ul elements
  • type and value attributes on li element
  • width attribute on pre element
• Additional elements in Transitional specification
  • menu (Deprecated. use CSS instead.) list (no substitute, though unordered list is recommended)
  • dir (Deprecated. use CSS instead.) list (no substitute, though unordered list is recommended)
  • isindex (Deprecated.) (element requires server-side support and is typically added to documents server-side, form and input elements can be used as a substitute)
  • applet (Deprecated. use the object element instead.)
• The language (Obsolete) attribute on script element (redundant with the type attribute).
• Frame related entities
  • iframe
  • noframes
  • target (Deprecated in the map, link and form elements.) attribute on a, client-side image-map (map), link, form and base elements

The Frameset version includes everything in the Transitional version, as well as the frameset element (used instead of body) and the frame element.

Frameset versus transitional

In addition to the above transitional differences, the frameset specifications (whether XHTML 1.0 or HTML 4.01) specify a different content model, with frameset replacing body, that contains either frame elements, or optionally noframes with a body.

Summary of specification versions

As this list demonstrates, the loose versions 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 specification primarily reined in all the various HTML implementations into a single clearly 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 was intended to be the first step in adding new features to the specification in a standards-body-based approach.

WHATWG HTML versus HTML5

The HTML Living Standard, which is developed by WHATWG, is the official version, while W3C HTML5 is no longer separate from WHATWG.

Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML.^[1] CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.^[2]

CSS is designed to enable the separation of presentation and content, including layout, colors, and fonts.^[3] This separation can improve content accessibility; provide more flexibility and control in the specification of presentation characteristics; enable multiple web pages to share formatting by specifying the relevant CSS in a separate .css file, which reduces complexity and repetition in the structural content; and enable the .css file to be cached to improve the page load speed between the pages that share the file and its formatting.

Separation of formatting and content also makes it feasible to present the same markup page in different styles for different rendering methods, such as on-screen, in print, by voice (via speech-based browser or screen reader), and on Braille-based tactile devices. CSS also has rules for alternate formatting if the content is accessed on a mobile device.^[4]

The name cascading comes from the specified priority scheme to determine which style rule applies if more than one rule matches a particular element. This cascading priority scheme is predictable.

The CSS specifications are maintained by the World Wide Web Consortium (W3C). Internet media type (MIME type) text/css is registered for use with CSS by RFC 2318 (March 1998). The W3C operates a free CSS validation service for CSS documents.^[5]

In addition to HTML, other markup languages support the use of CSS including XHTML, plain XML, SVG, and XUL.

Syntax[edit]

CSS has a simple syntax and uses a number of English keywords to specify the names of various style properties.

A style sheet consists of a list of rules. Each rule or rule-set consists of one or more selectors, and a declaration block.

Selector[edit]

In CSS, selectors declare which part of the markup a style applies to by matching tags and attributes in the markup itself.

Selectors may apply to the following:

• all elements of a specific type, e.g. the second-level headers h2
• elements specified by attribute, in particular:
  • id: an identifier unique within the document, identified with a hash prefix e.g. #id
  • class: an identifier that can annotate multiple elements in a document, identified with a period prefix e.g. .classname
• elements depending on how they are placed relative to others in the document tree.

Classes and IDs are case-sensitive, start with letters, and can include alphanumeric characters, hyphens, and underscores. A class may apply to any number of instances of any elements. An ID may only be applied to a single element.

Pseudo-classes are used in CSS selectors to permit formatting based on information that is not contained in the document tree. One example of a widely used pseudo-class is :hover, which identifies content only when the user "points to" the visible element, usually by holding the mouse cursor over it. It is appended to a selector as in a:hover or #elementid:hover. A pseudo-class classifies document elements, such as :link or :visited, whereas a pseudo-element makes a selection that may consist of partial elements, such as ::first-line or ::first-letter.^[6]

Selectors may be combined in many ways to achieve great specificity and flexibility.^[7] Multiple selectors may be joined in a spaced list to specify elements by location, element type, id, class, or any combination thereof. The order of the selectors is important. For example, div .myClass {color: red;} applies to all elements of class myClass that are inside div elements, whereas .myClass div {color: red;} applies to all div elements that are inside elements of class myClass. This is not to be confused with concatenated identifiers such as div.myClass {color: red;} which applies to div elements of class myClass.

The following table provides a summary of selector syntax indicating usage and the version of CSS that introduced it.^[8]

Declaration block[edit]

A declaration block consists of a list of declarations in braces. Each declaration itself consists of a property, a colon (:), and a value. If there are multiple declarations in a block, a semi-colon (;) must be inserted to separate each declaration. An optional semi-colon after the last (or single) declaration may be used.^[9]

Properties are specified in the CSS standard. Each property has a set of possible values. Some properties can affect any type of element, and others apply only to particular groups of elements.^[10][11]

Values may be keywords, such as "center" or "inherit", or numerical values, such as 200px (200 pixels), 50vw (50 percent of the viewport width) or 80% (80 percent of the parent element's width). Color values can be specified with keywords (e.g. "red"), hexadecimal values (e.g. #FF0000, also abbreviated as #F00), RGB values on a 0 to 255 scale (e.g. rgb(255, 0, 0)), RGBA values that specify both color and alpha transparency (e.g. rgba(255, 0, 0, 0.8)), or HSL or HSLA values (e.g. hsl(000, 100%, 50%), hsla(000, 100%, 50%, 80%)).^[12]

Length units[edit]

Non-zero numeric values representing linear measures must include a length unit, which is either an alphabetic code or abbreviation, as in 200px or 50vw; or a percentage sign, as in 80%. Some units – cm (centimetre); in (inch); mm (millimetre); pc (pica); and pt (point) – are absolute, which means that the rendered dimension does not depend upon the structure of the page; others – em (em); ex (ex) and px (pixel) – are relative, which means that factors such as the font size of a parent element can affect the rendered measurement. These eight units were a feature of CSS 1^[13] and retained in all subsequent revisions. The proposed CSS Values and Units Module Level 3 will, if adopted as a W3C Recommendation, provide seven further length units: ch; Q; rem; vh; vmax; vmin; and vw.^[14]

Use[edit]

Before CSS, nearly all presentational attributes of HTML documents were contained within the HTML markup. All font colors, background styles, element alignments, borders and sizes had to be explicitly described, often repeatedly, within the HTML. CSS lets authors move much of that information to another file, the style sheet, resulting in considerably simpler HTML.

For example, headings (h1 elements), sub-headings (h2), sub-sub-headings (h3), etc., are defined structurally using HTML. In print and on the screen, choice of font, size, color and emphasis for these elements is presentational.

Before CSS, document authors who wanted to assign such typographic characteristics to, say, all h2 headings had to repeat HTML presentational markup for each occurrence of that heading type. This made documents more complex, larger, and more error-prone and difficult to maintain. CSS allows the separation of presentation from structure. CSS can define color, font, text alignment, size, borders, spacing, layout and many other typographic characteristics, and can do so independently for on-screen and printed views. CSS also defines non-visual styles, such as reading speed and emphasis for aural text readers. The W3C has now deprecated the use of all presentational HTML markup.^[15]

For example, under pre-CSS HTML, a heading element defined with red text would be written as:

<h1><font color="red">Chapter 1.</font></h1>

Using CSS, the same element can be coded using style properties instead of HTML presentational attributes:

<h1 style="color: red;">Chapter 1.</h1>

The advantages of this may not be immediately clear but the power of CSS becomes more apparent when the style properties are placed in an internal style element or, even better, an external CSS file. For example, suppose the document contains the style element:

<style>
    h1 {
        color: red;
    }
</style>

All h1 elements in the document will then automatically become red without requiring any explicit code. If the author later wanted to make h1 elements blue instead, this could be done by changing the style element to:

<style>
    h1 {
        color: blue;
    }
</style>

rather than by laboriously going through the document and changing the color for each individual h1 element.

The styles can also be placed in an external CSS file, as described below, and loaded using syntax similar to:

<link href="path/to/file.css" rel="stylesheet" type="text/css">

This further decouples the styling from the HTML document and makes it possible to restyle multiple documents by simply editing a shared external CSS file.

Sources[edit]

CSS information can be provided from various sources. These sources can be the web browser, the user, and the author. The information from the author can be further classified into inline, media type, importance, selector specificity, rule order, inheritance, and property definition. CSS style information can be in a separate document, or it can be embedded into an HTML document. Multiple style sheets can be imported. Different styles can be applied depending on the output device being used; for example, the screen version can be quite different from the printed version, so that authors can tailor the presentation appropriately for each medium.

The style sheet with the highest priority controls the content display. Declarations not set in the highest priority source are passed on to a source of lower priority, such as the user agent style. The process is called cascading.

One of the goals of CSS is to allow users greater control over presentation. Someone who finds red italic headings difficult to read may apply a different style sheet. Depending on the browser and the web site, a user may choose from various style sheets provided by the designers, or may remove all added styles and view the site using the browser's default styling, or may override just the red italic heading style without altering other attributes.

Specificity[edit]

Specificity refers to the relative weights of various rules.^[16] It determines which styles apply to an element when more than one rule could apply. Based on specification, a simple selector (e.g. H1) has a specificity of 1, class selectors have a specificity of 1,0, and ID selectors a specificity of 1,0,0. Because the specificity values do not carry over as in the decimal system, commas are used to separate the "digits"^[17] (a CSS rule having 11 elements and 11 classes would have a specificity of 11,11, not 121).

Thus the following rules selectors result in the indicated specificity:

Examples[edit]

Consider this HTML fragment:

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <style>
            #xyz { color: blue; }
        </style>
    </head>
    <body>
        <p id="xyz" style="color: green;">To demonstrate specificity</p>
    </body>
</html>

In the above example, the declaration in the style attribute overrides the one in the <style> element because it has a higher specificity, and thus, the paragraph appears green.

Inheritance[edit]

Inheritance is a key feature in CSS; it relies on the ancestor-descendant relationship to operate. Inheritance is the mechanism by which properties are applied not only to a specified element, but also to its descendants.^[16] Inheritance relies on the document tree, which is the hierarchy of XHTML elements in a page based on nesting. Descendant elements may inherit CSS property values from any ancestor element enclosing them. In general, descendant elements inherit text-related properties, but their box-related properties are not inherited. Properties that can be inherited are color, font, letter-spacing, line-height, list-style, text-align, text-indent, text-transform, visibility, white-space and word-spacing. Properties that cannot be inherited are background, border, display, float and clear, height, and width, margin, min- and max-height and -width, outline, overflow, padding, position, text-decoration, vertical-align and z-index.

Inheritance can be used to avoid declaring certain properties over and over again in a style sheet, allowing for shorter CSS.

Inheritance in CSS is not the same as inheritance in class-based programming languages, where it is possible to define class B as "like class A, but with modifications".^[18] With CSS, it is possible to style an element with "class A, but with modifications". However, it is not possible to define a CSS class B like that, which could then be used to style multiple elements without having to repeat the modifications.

Example[edit]

Given the following style sheet:

h1 {
   color: pink;
}

Suppose there is an h1 element with an emphasizing element (em) inside:

<h1>
   This is to <em>illustrate</em> inheritance
</h1>

If no color is assigned to the em element, the emphasized word "illustrate" inherits the color of the parent element, h1. The style sheet h1 has the color pink, hence, the em element is likewise pink.

Whitespace[edit]

Whitespace between properties and selectors is ignored. This code snippet:

body{overflow:hidden;background:#000000;background-image:url(images/bg.gif);background-repeat:no-repeat;background-position:left top;}

is functionally equivalent to this one:

body {
   overflow: hidden;
   background-color: #000000;
   background-image: url(images/bg.gif);
   background-repeat: no-repeat;
   background-position: left top;
}

One common way to format CSS for readability is to indent each property and give it its own line. In addition to formatting CSS for readability, shorthand properties can be used to write out the code faster, which also gets processed more quickly when being rendered:^[19]

body {
   overflow: hidden;
   background: #000 url(images/bg.gif) no-repeat left top;
}

Positioning[edit]

CSS 2.1 defines three positioning schemes:

Normal flow

Inline items are laid out in the same way as the letters in words in text, one after the other across the available space until there is no more room, then starting a new line below. Block items stack vertically, like paragraphs and like the items in a bulleted list. Normal flow also includes relative positioning of block or inline items, and run-in boxes.

Floats

A floated item is taken out of the normal flow and shifted to the left or right as far as possible in the space available. Other content then flows alongside the floated item.

Absolute positioning

An absolutely positioned item has no place in, and no effect on, the normal flow of other items. It occupies its assigned position in its container independently of other items.^[20]

Position property[edit]

There are five possible values of the position property. If an item is positioned in any way other than static, then the further properties top, bottom, left, and right are used to specify offsets and positions.The element having position static is not affected by the top, bottom , left or right properties.

Static

The default value places the item in the normal flow

Relative

The item is placed in the normal flow, and then shifted or offset from that position. Subsequent flow items are laid out as if the item had not been moved.

Absolute

Specifies absolute positioning. The element is positioned in relation to its nearest non-static ancestor.

Fixed

The item is absolutely positioned in a fixed position on the screen even as the rest of the document is scrolled^[20]

Float and clear[edit]

The float property may have one of three values. Absolutely positioned or fixed items cannot be floated. Other elements normally flow around floated items, unless they are prevented from doing so by their clear property.

left

The item floats to the left of the line that it would have appeared in; other items may flow around its right side.

right

The item floats to the right of the line that it would have appeared in; other items may flow around its left side.

JavaScript

JavaScript (/ˈdʒɑːvəskrɪpt/),[10] often abbreviated JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS.[11] Over 97% of websites use JavaScript on the client side for web page behavior,[12] often incorporating third-party libraries.[13] All major web browsers have a dedicated JavaScript engine to execute the code on users' devices. JavaScript is a high-level, often just-in-time compiled language that conforms to the ECMAScript standard.[14] It has dynamic typing, prototype-based object-orientation, and first-class functions. It is multi-paradigm, supporting event-driven, functional, and imperative programming styles. It has application programming interfaces (APIs) for working with text, dates, regular expressions, standard data structures, and the Document Object Model (DOM). The ECMAScript standard does not include any input/output (I/O), such as networking, storage, or graphics facilities. In practice, the web browser or other runtime system provides JavaScript APIs for I/O. JavaScript engines were originally used only in web browsers, but are now core components of some servers and a variety of applications. The most popular runtime system for this usage is Node.js. Although Java and JavaScript are similar in name, syntax, and respective standard libraries, the two languages are distinct and differ greatly in design.

History[edit]

Creation at Netscape[edit]

The first web browser with a graphical user interface, Mosaic, was released in 1993. Accessible to non-technical people, it played a prominent role in the rapid growth of the nascent World Wide Web.^[15] The lead developers of Mosaic then founded the Netscape corporation, which released a more polished browser, Netscape Navigator, in 1994. This quickly became the most-used.^[16][17]

During these formative years of the Web, web pages could only be static, lacking the capability for dynamic behavior after the page was loaded in the browser. There was a desire in the burgeoning web development scene to remove this limitation, so in 1995, Netscape decided to add a scripting language to Navigator. They pursued two routes to achieve this: collaborating with Sun Microsystems to embed the Java programming language, while also hiring Brendan Eich to embed the Scheme language.^[6]

Netscape management soon decided that the best option was for Eich to devise a new language, with syntax similar to Java and less like Scheme or other extant scripting languages.^[5][6] Although the new language and its interpreter implementation were called LiveScript when first shipped as part of a Navigator beta in September 1995, the name was changed to JavaScript for the official release in December.^[6][1][18]

The choice of the JavaScript name has caused confusion, implying that it is directly related to Java. At the time, the dot-com boom had begun and Java was the hot new language, so Eich considered the JavaScript name a marketing ploy by Netscape.^[19]

Adoption by Microsoft[edit]

Microsoft debuted Internet Explorer in 1995, leading to a browser war with Netscape. On the JavaScript front, Microsoft reverse-engineered the Navigator interpreter to create its own, called JScript.^[20]

JScript was first released in 1996, alongside initial support for CSS and extensions to HTML. Each of these implementations was noticeably different from their counterparts in Navigator.^[21][22] These differences made it difficult for developers to make their websites work well in both browsers, leading to widespread use of "best viewed in Netscape" and "best viewed in Internet Explorer" logos for several years.^[21][23]

The rise of JScript[edit]

In November 1996, Netscape submitted JavaScript to Ecma International, as the starting point for a standard specification that all browser vendors could conform to. This led to the official release of the first ECMAScript language specification in June 1997.

The standards process continued for a few years, with the release of ECMAScript 2 in June 1998 and ECMAScript 3 in December 1999. Work on ECMAScript 4 began in 2000.^[20]

Meanwhile, Microsoft gained an increasingly dominant position in the browser market. By the early 2000s, Internet Explorer's market share reached 95%.^[24] This meant that JScript became the de facto standard for client-side scripting on the Web.

Microsoft initially participated in the standards process and implemented some proposals in its JScript language, but eventually it stopped collaborating on Ecma work. Thus ECMAScript 4 was mothballed.

Growth and standardization[edit]

During the period of Internet Explorer dominance in the early 2000s, client-side scripting was stagnant. This started to change in 2004, when the successor of Netscape, Mozilla, released the Firefox browser. Firefox was well received by many, taking significant market share from Internet Explorer.^[25]

In 2005, Mozilla joined ECMA International, and work started on the ECMAScript for XML (E4X) standard. This led to Mozilla working jointly with Macromedia (later acquired by Adobe Systems), who were implementing E4X in their ActionScript 3 language, which was based on an ECMAScript 4 draft. The goal became standardizing ActionScript 3 as the new ECMAScript 4. To this end, Adobe Systems released the Tamarin implementation as an open source project. However, Tamarin and ActionScript 3 were too different from established client-side scripting, and without cooperation from Microsoft, ECMAScript 4 never reached fruition.

Meanwhile, very important developments were occurring in open-source communities not affiliated with ECMA work. In 2005, Jesse James Garrett released a white paper in which he coined the term Ajax and described a set of technologies, of which JavaScript was the backbone, to create web applications where data can be loaded in the background, avoiding the need for full page reloads. This sparked a renaissance period of JavaScript, spearheaded by open-source libraries and the communities that formed around them. Many new libraries were created, including jQuery, Prototype, Dojo Toolkit, and MooTools.

Google debuted its Chrome browser in 2008, with the V8 JavaScript engine that was faster than its competition.^[26][27] The key innovation was just-in-time compilation (JIT),^[28] so other browser vendors needed to overhaul their engines for JIT.^[29]

In July 2008, these disparate parties came together for a conference in Oslo. This led to the eventual agreement in early 2009 to combine all relevant work and drive the language forward. The result was the ECMAScript 5 standard, released in December 2009.

Reaching maturity[edit]

Ambitious work on the language continued for several years, culminating in an extensive collection of additions and refinements being formalized with the publication of ECMAScript 6 in 2015.^[30]

The creation of Node.js in 2009 by Ryan Dahl sparked a significant increase in the usage of JavaScript outside of web browsers. Node combines the V8 engine, an event loop, and I/O APIs, thereby providing a stand-alone JavaScript runtime system.^[31][32] As of 2018, Node had been used by millions of developers,^[33] and npm had the most modules of any package manager in the world.^[34]

The ECMAScript draft specification is currently maintained openly on GitHub, and editions are produced via regular annual snapshots.^[35] Potential revisions to the language are vetted through a comprehensive proposal process.^[36][37] Now, instead of edition numbers, developers check the status of upcoming features individually.^[35]

The current JavaScript ecosystem has many libraries and frameworks, established programming practices, and substantial usage of JavaScript outside of web browsers. Plus, with the rise of single-page applications and other JavaScript-heavy websites, several transpilers have been created to aid the development process.^[38]

Trademark[edit]

"JavaScript" is a trademark of Oracle Corporation in the United States.^[39][40]

Website client-side usage[edit]

JavaScript is the dominant client-side scripting language of the Web, with 97% of websites using it for this purpose.^[12] Scripts are embedded in or included from HTML documents and interact with the DOM. All major web browsers have a built-in JavaScript engine that executes the code on the user's device.

Examples of scripted behavior[edit]

• Loading new web page content without reloading the page, via Ajax or a WebSocket. For example, users of social media can send and receive messages without leaving the current page.
• Web page animations, such as fading objects in and out, resizing, and moving them.
• Playing browser games.
• Controlling the playback of streaming media.
• Generating pop-up ads.
• Validating input values of a web form before the data is sent to a web server.
• Logging data about the user's behavior then sending it to a server. The website owner can use this data for analytics, ad tracking, and personalization.
• Redirecting a user to another page.

Libraries and frameworks[edit]

Over 80% of websites use a third-party JavaScript library or web framework for their client-side scripting.^[13]

jQuery is by far the most popular library, used by over 75% of websites.^[13] Facebook created the React library for its website and later released it as open source; other sites, including Twitter, now use it. Likewise, the Angular framework created by Google for its websites, including YouTube and Gmail, is now an open source project used by others.^[13]

In contrast, the term "Vanilla JS" has been coined for websites not using any libraries or frameworks, instead relying entirely on standard JavaScript functionality.^[41]

Other usage[edit]

The use of JavaScript has expanded beyond its web browser roots. JavaScript engines are now embedded in a variety of other software systems, both for server-side website deployments and non-browser applications.

Initial attempts at promoting server-side JavaScript usage were Netscape Enterprise Server and Microsoft's Internet Information Services,^[42][43] but they were small niches.^[44] Server-side usage eventually started to grow in the late 2000s, with the creation of Node.js and other approaches.^[44]

Electron, Cordova, React Native, and other application frameworks have been used to create many applications with behavior implemented in JavaScript. Other non-browser applications include Adobe Acrobat support for scripting PDF documents^[45] and GNOME Shell extensions written in JavaScript.^[46]

JavaScript has recently begun to appear in some embedded systems, usually by leveraging Node.js.^[47][48][49]

Features[edit]

The following features are common to all conforming ECMAScript implementations unless explicitly specified otherwise.

Imperative and structured[edit]

JavaScript supports much of the structured programming syntax from C (e.g., if statements, while loops, switch statements, do while loops, etc.). One partial exception is scoping: originally JavaScript only had function scoping with var; then block scoping was added in ECMAScript 2015 with the keywords let and const. Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, which allow semicolons (which terminate statements) to be omitted.^[50]

Weakly typed[edit]

JavaScript is weakly typed, which means certain types are implicitly cast depending on the operation used.^[51]

• The binary + operator casts both operands to a string unless both operands are numbers. This is because the addition operator doubles as a concatenation operator
• The binary - operator always casts both operands to a number
• Both unary operators (+, -) always cast the operand to a number

Values are cast to strings like the following:^[51]

• Strings are left as-is
• Numbers are converted to their string representation
• Arrays have their elements cast to strings after which they are joined by commas (,)
• Other objects are converted to the string [object Object] where Object is the name of the constructor of the object

Values are cast to numbers by casting to strings and then casting the strings to numbers. These processes can be modified by defining toString and valueOf functions on the prototype for string and number casting respectively.

JavaScript has received criticism for the way it implements these conversions as the complexity of the rules can be mistaken for inconsistency.^[52][51] For example, when adding a number to a string, the number will be cast to a string before performing concatenation, but when subtracting a number from a string, the string is cast to a number before performing subtraction.

JavaScript type conversions

left operand	operator	right operand	result
[] (empty array)	+	[] (empty array)	"" (empty string)
[] (empty array)	+	{} (empty object)	"[object Object]" (string)
false (boolean)	+	[] (empty array)	"false" (string)
"123"(string)	+	1 (number)	"1231" (string)
"123" (string)	-	1 (number)	122 (number)
"123" (string)	-	"abc" (string)	NaN (number)

Often also mentioned is {} + [] resulting in 0 (number). This is misleading: the {} is interpreted as an empty code block instead of an empty object, and the empty array is cast to a number by the remaining unary + operator. If you wrap the expression in parentheses ({} + []) the curly brackets are interpreted as an empty object and the result of the expression is "[object Object]" as expected.^[51]

Dynamic[edit]

Typing

JavaScript is dynamically typed like most other scripting languages. A type is associated with a value rather than an expression. For example, a variable initially bound to a number may be reassigned to a string.^[53] JavaScript supports various ways to test the type of objects, including duck typing.^[54]

Run-time evaluation

JavaScript includes an eval function that can execute statements provided as strings at run-time.

Object-orientation (prototype-based)[edit]

Prototypal inheritance in JavaScript is described by Douglas Crockford as:

You make prototype objects, and then ... make new instances. Objects are mutable in JavaScript, so we can augment the new instances, giving them new fields and methods. These can then act as prototypes for even newer objects. We don't need classes to make lots of similar objects... Objects inherit from objects. What could be more object oriented than that?^[55]

In JavaScript, an object is an associative array, augmented with a prototype (see below); each key provides the name for an object property, and there are two syntactical ways to specify such a name: dot notation (obj.x = 10) and bracket notation (obj['x'] = 10). A property may be added, rebound, or deleted at run-time. Most properties of an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using a for...in loop.

Prototypes

JavaScript uses prototypes where many other object-oriented languages use classes for inheritance.^[56] It is possible to simulate many class-based features with prototypes in JavaScript.^[57]

Functions as object constructors

Functions double as object constructors, along with their typical role. Prefixing a function call with new will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from the Object prototype).^[58] ECMAScript 5 offers the Object.create method, allowing explicit creation of an instance without automatically inheriting from the Object prototype (older environments can assign the prototype to null).^[59] The constructor's prototype property determines the object used for the new object's internal prototype. New methods can be added by modifying the prototype of the function used as a constructor. JavaScript's built-in constructors, such as Array or Object, also have prototypes that can be modified. While it is possible to modify the Object prototype, it is generally considered bad practice because most objects in JavaScript will inherit methods and properties from the Object prototype, and they may not expect the prototype to be modified.^[60]

Functions as methods

Unlike many object-oriented languages, there is no distinction between a function definition and a method definition. Rather, the distinction occurs during function calling; when a function is called as a method of an object, the function's local this keyword is bound to that object for that invocation.

Functional[edit]

A function is first-class; a function is considered to be an object.^[61] As such, a function may have properties and methods, such as .call() and .bind().^[62] A nested function is a function defined within another function. It is created each time the outer function is invoked. In addition, each nested function forms a lexical closure: the lexical scope of the outer function (including any constant, local variable, or argument value) becomes part of the internal state of each inner function object, even after execution of the outer function concludes.^[63] JavaScript also supports anonymous functions.

Delegative[edit]

JavaScript supports implicit and explicit delegation.

Functions as roles (Traits and Mixins)

JavaScript natively supports various function-based implementations of Role^[64] patterns like Traits^[65][66] and Mixins.^[67] Such a function defines additional behavior by at least one method bound to the this keyword within its function body. A Role then has to be delegated explicitly via call or apply to objects that need to feature additional behavior that is not shared via the prototype chain.

Object composition and inheritance

Whereas explicit function-based delegation does cover composition in JavaScript, implicit delegation already happens every time the prototype chain is walked in order to, e.g., find a method that might be related to but is not directly owned by an object. Once the method is found it gets called within this object's context. Thus inheritance in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.

Miscellaneous[edit]

JS is a zero-index language.

Run-time environment

JavaScript typically relies on a run-time environment (e.g., a web browser) to provide objects and methods by which scripts can interact with the environment (e.g., a web page DOM). These environments are single-threaded. JavaScript also relies on the run-time environment to provide the ability to include/import scripts (e.g., HTML <script> elements). This is not a language feature per se, but it is common in most JavaScript implementations. JavaScript processes messages from a queue one at a time. JavaScript calls a function associated with each new message, creating a call stack frame with the function's arguments and local variables. The call stack shrinks and grows based on the function's needs. When the call stack is empty upon function completion, JavaScript proceeds to the next message in the queue. This is called the event loop, described as "run to completion" because each message is fully processed before the next message is considered. However, the language's concurrency model describes the event loop as non-blocking: program input/output is performed using events and callback functions. This means, for instance, that JavaScript can process a mouse click while waiting for a database query to return information.^[68]

Variadic functions

An indefinite number of parameters can be passed to a function. The function can access them through formal parameters and also through the local arguments object. Variadic functions can also be created by using the bind method.

Array and object literals

Like many scripting languages, arrays and objects (associative arrays in other languages) can each be created with a succinct shortcut syntax. In fact, these literals form the basis of the JSON data format.

Regular expressions

JavaScript also supports regular expressions in a manner similar to Perl, which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built-in string functions.^[69]

Promises and Async/await

JavaScript supports promises and Async/await for handling asynchronous operations. A built-in Promise object provides functionality for handling promises and associating handlers with an asynchronous action's eventual result. Recently, combinator methods were introduced in the JavaScript specification, which allows developers to combine multiple JavaScript promises and do operations based on different scenarios. The methods introduced are: Promise.race, Promise.all, Promise.allSettled and Promise.any. Async/await allows an asynchronous, non-blocking function to be structured in a way similar to an ordinary synchronous function. Asynchronous, non-blocking code can be written, with minimal overhead, structured similar to traditional synchronous, blocking code.

Vendor-specific extensions[edit]

Historically, some JavaScript engines supported these non-standard features:

• conditional catch clauses (like Java)
• array comprehensions and generator expressions (like Python)
• concise function expressions (function(args) expr; this experimental syntax predated arrow functions)
• ECMAScript for XML (E4X), an extension that adds native XML support to ECMAScript (unsupported in Firefox since version 21^[70])

Syntax[edit]

Main article: JavaScript syntax

Simple examples[edit]

Variables in JavaScript can be defined using either the var,^[71] let^[72] or const^[73] keywords.

// Declares a function-scoped variable named `x`, and implicitly assigns the
// special value `undefined` to it. Variables without value are automatically
// set to undefined.
var x;

// Variables can be manually set to `undefined` like so
var x2 = undefined;

// Declares a block-scoped variable named `y`, and implicitly sets it to
// `undefined`. The `let` keyword was introduced in ECMAScript 2015.
let y;

// Declares a block-scoped, un-reassignable variable named `z`, and sets it to
// a string literal. The `const` keyword was also introduced in ECMAScript 2015,
// and must be explicitly assigned to.

// The keyword `const` means constant, hence the variable cannot be reassigned
// as the value is `constant`.
const z = "this value cannot be reassigned!";

// Declares a variable named `myNumber`, and assigns a number literal (the value
// `2`) to it.
let myNumber = 2;

// Reassigns `myNumber`, setting it to a string literal (the value `"foo"`).
// JavaScript is a dynamically-typed language, so this is legal.
myNumber = "foo";

Note the comments in the example above, all of which were preceded with two forward slashes.

There is no built-in Input/output functionality in JavaScript; the run-time environment provides that. The ECMAScript specification in edition 5.1 mentions:^[74]

indeed, there are no provisions in this specification for input of external data or output of computed results.

However, most runtime environments have a console object^[75] that can be used to print output. Here is a minimalist Hello World program in JavaScript:

console.log("Hello World!");

A simple recursive function:

function factorial(n) {
    if (n === 0)
        return 1; // 0! = 1

    return n * factorial(n - 1);
}

factorial(3); // returns 6

An anonymous function (or lambda):

function counter() {
    let count = 0;

    return function() {
        return ++count;
    };
}

let x = counter();
x(); // returns 1
x(); // returns 2
x(); // returns 3

This example shows that, in JavaScript, function closures capture their non-local variables by reference.

Arrow functions were first introduced in 6th Edition - ECMAScript 2015. They shorten the syntax for writing functions in JavaScript. Arrow functions are anonymous, so a variable is needed to refer to them in order to invoke them after their creation.

Example of arrow function:

// Arrow functions let us omit the `function` keyword.
// Here `long_example` points to an anonymous function value.
const long_example = (input1, input2) => {
    console.log("Hello, World!");
    const output = input1 + input2;

    return output;
};

// If there are no braces, the arrow function simply returns the expression
// So here it's (input1 + input2)
const short_example = (input1, input2) => input1 + input2;

long_example(2, 3); // Prints "Hello, World!" and returns 5
short_example(2, 5);  // Returns 7

// If an arrow function only has one parameter, the parentheses can be removed.
const no_parentheses = input => input + 2;

no_parentheses(3); // Returns 5

In JavaScript, objects are created in the same way as functions; this is known as a function object.

Object example:

function Ball(r) {
    this.radius = r; // the "r" argument is local to the ball object
    this.area = Math.PI * (r ** 2); // parentheses don't do anything but clarify

    // objects can contain functions ("method")
    this.show = function() {
        drawCircle(this.radius); // references another function (that draws a circle)
    };
}

let myBall = new Ball(5); // creates a new instance of the ball object with radius 5
myBall.radius++; // object properties can usually be modified from the outside
myBall.show(); // using the inherited "show" function

Variadic function demonstration (arguments is a special variable):^[76]

function sum() {
    let x = 0;

    for (let i = 0; i < arguments.length; ++i)
        x += arguments[i];

    return x;
}

sum(1, 2); // returns 3
sum(1, 2, 3); // returns 6

Immediately-invoked function expressions are often used to create closures. Closures allow gathering properties and methods in a namespace and making some of them private:

let counter = (function() {
    let i = 0; // private property

    return {   // public methods
        get: function() {
            alert(i);
        },
        set: function(value) {
            i = value;
        },
        increment: function() {
            alert(++i);
        }
    };
})(); // module

counter.get();      // shows 0
counter.set(6);
counter.increment(); // shows 7
counter.increment(); // shows 8

Exporting and Importing modules in JavaScript^[77]

Export example:

/* mymodule.js */
// This function remains private, as it is not exported
let sum = (a, b) => {
    return a + b;
}

// Export variables
export let name = 'Alice';
export let age = 23;

// Export named functions
export function add(num1, num2) {
    return num1 + num2;
}

// Export class
export class Multiplication {
    constructor(num1, num2) {
        this.num1 = num1;
        this.num2 = num2;
    }

    add() {
        return sum(this.num1, this.num2);
    }
}

Import example:

// Import one property
import { add } from './mymodule.js';

console.log(add(1, 2)); // 3

// Import multiple properties
import { name, age } from './mymodule.js';
console.log(name, age);
//> "Alice", 23

// Import all properties from a module
import * from './module.js'
console.log(name, age);
//> "Alice", 23
console.log(add(1,2));
//> 3

More advanced example[edit]

This sample code displays various JavaScript features.

/* Finds the lowest common multiple (LCM) of two numbers */
function LCMCalculator(x, y) { // constructor function
    const checkInt = function(x) { // inner function
        if (x % 1 !== 0)
            throw new TypeError(x + "is not an integer"); // var a =  mouseX

        return x;
    };

    this.a = checkInt(x)
    //   semicolons   ^^^^  are optional, a newline is enough
    this.b = checkInt(y);
}
// The prototype of object instances created by a constructor is
// that constructor's "prototype" property.
LCMCalculator.prototype = { // object literal
    constructor: LCMCalculator, // when reassigning a prototype, set the constructor property appropriately
    gcd: function() { // method that calculates the greatest common divisor
        // Euclidean algorithm:
        let a = Math.abs(this.a), b = Math.abs(this.b), t;

        if (a < b) {
            // swap variables
            // t = b; b = a; a = t;
            [a, b] = [b, a]; // swap using destructuring assignment (ES6)
        }

        while (b !== 0) {
            t = b;
            b = a % b;
            a = t;
        }

        // Only need to calculate GCD once, so "redefine" this method.
        // (Actually not redefinition—it's defined on the instance itself,
        // so that this.gcd refers to this "redefinition" instead of LCMCalculator.prototype.gcd.
        // Note that this leads to a wrong result if the LCMCalculator object members "a" and/or "b" are altered afterwards.)
        // Also, 'gcd' === "gcd", this['gcd'] === this.gcd
        this['gcd'] = function() {
            return a;
        };

        return a;
    },

    // Object property names can be specified by strings delimited by double (") or single (') quotes.
    "lcm": function() {
        // Variable names do not collide with object properties, e.g., |lcm| is not |this.lcm|.
        // not using |this.a*this.b| to avoid FP precision issues
        let lcm = this.a / this.gcd() * this.b;

        // Only need to calculate lcm once, so "redefine" this method.
        this.lcm = function() {
            return lcm;
        };

        return lcm;
    },

    // Methods can also be declared using es6 syntax
    toString() {
        // Using both es6 template literals and the (+) operator to concatenate values
        return `LCMCalculator: a = ${this.a}, b = ` + this.b;
    }
};

// Define generic output function; this implementation only works for Web browsers
function output(x) {
    document.body.appendChild(document.createTextNode(x));
    document.body.appendChild(document.createElement('br'));
}

// Note: Array's map() and forEach() are defined in JavaScript 1.6.
// They are used here to demonstrate JavaScript's inherent functional nature.
[
    [25, 55],
    [21, 56],
    [22, 58],
    [28, 56]
].map(function(pair) { // array literal + mapping function
    return new LCMCalculator(pair[0], pair[1]);
}).sort((a, b) => a.lcm() - b.lcm()) // sort with this comparative function; => is a shorthand form of a function, called "arrow function"
    .forEach(printResult);

function printResult(obj) {
    output(obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());
}

The following output should be displayed in the browser window.

LCMCalculator: a = 28, b = 56, gcd = 28, lcm = 56
LCMCalculator: a = 21, b = 56, gcd = 7, lcm = 168
LCMCalculator: a = 25, b = 55, gcd = 5, lcm = 275
LCMCalculator: a = 22, b = 58, gcd = 2, lcm = 638