Ajax (programming)

From Wikipedia, the free encyclopedia

Asynchronous JavaScript and XML

First appeared	March 1999
Filename extensions	.js
File formats	JavaScript
Influenced by
JavaScript and XML

Ajax (also AJAX /ˈeɪdʒæks/; short for "Asynchronous JavaScript And XML") is a set of Web development techniques using many web technologies on the client side to create asynchronous Web applications. With Ajax, web applications can send and retrieve data from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows web pages, and by extension web applications, to change content dynamically without the need to reload the entire page. In practice, modern implementations commonly utilize JSON instead of XML due to the advantages of JSON being native to JavaScript.

Ajax is not a single technology, but rather a group of technologies. HTML and CSS can be used in combination to mark up and style information. The webpage can then be modified by JavaScript to dynamically display – and allow the user to interact with — the new information. The built-in XMLHttpRequest object within JavaScript is commonly used to execute Ajax on webpages allowing websites to load content onto the screen without refreshing the page. Ajax is not a new technology, or different language, just existing technologies used in new ways.

History

In the early-to-mid 1990s, most Web sites were based on complete HTML pages. Each user action required that a complete new page be loaded from the server. This process was inefficient, as reflected by the user experience: all page content disappeared, then the new page appeared. Each time the browser reloaded a page because of a partial change, all of the content had to be re-sent, even though only some of the information had changed. This placed additional load on the server and made bandwidth a limiting factor on performance. 

In 1996, the iframe tag was introduced by Internet Explorer; like the object element, it can load or fetch content asynchronously. In 1998, the Microsoft Outlook Web Access team developed the concept behind the XMLHttpRequest scripting object. It appeared as XMLHTTP in the second version of the MSXML library, which shipped with Internet Explorer 5.0 in March 1999.

The functionality of the XMLHTTP ActiveX control in IE 5 was later implemented by Mozilla, Safari, Opera and other browsers as the XMLHttpRequest JavaScript object. Microsoft adopted the native XMLHttpRequest model as of Internet Explorer 7. The ActiveX version is still supported in Internet Explorer, but not in Microsoft Edge. The utility of these background HTTP requests and asynchronous Web technologies remained fairly obscure until it started appearing in large scale online applications such as Outlook Web Access (2000) and Oddpost (2002). 

Google made a wide deployment of standards-compliant, cross browser Ajax with Gmail (2004) and Google Maps (2005). In October 2004 Kayak.com's public beta release was among the first large-scale e-commerce uses of what their developers at that time called "the xml http thing". This increased interest in AJAX among web program developers. 

The term Ajax was publicly used on 18 February 2005 by Jesse James Garrett in an article titled Ajax: A New Approach to Web Applications, based on techniques used on Google pages.

On 5 April 2006, the World Wide Web Consortium (W3C) released the first draft specification for the XMLHttpRequest object in an attempt to create an official Web standard. The latest draft of the XMLHttpRequest object was published on 30 January 2014.

Technologies

The conventional model for a Web Application versus an application using Ajax

 

The term Ajax has come to represent a broad group of Web technologies that can be used to implement a Web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax, Jesse James Garrett explained that the following technologies are incorporated:

• HTML (or XHTML) and CSS for presentation
• The Document Object Model (DOM) for dynamic display of and interaction with data
• JSON or XML for the interchange of data, and XSLT for its manipulation
• The XMLHttpRequest object for asynchronous communication
• JavaScript to bring these technologies together

Since then, however, there have been a number of developments in the technologies used in an Ajax application, and in the definition of the term Ajax itself. XML is no longer required for data interchange and, therefore, XSLT is no longer required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange, although other formats such as preformatted HTML or plain text can also be used. A variety of popular JavaScript libraries, including JQuery, include abstractions to assist in executing Ajax requests.

Asynchronous HTML and HTTP (AHAH) involves using XMLHTTPRequest to retrieve (X)HTML fragments, which are then inserted directly into the Web page.

Drawbacks

• Any user whose browser does not support JavaScript or XMLHttpRequest, or has this functionality disabled, will not be able to properly use pages that depend on Ajax. Simple devices (such as smartphones and PDAs) may not support the required technologies. The only way to let the user carry out functionality is to fall back to non-JavaScript methods. This can be achieved by making sure links and forms can be resolved properly and not relying solely on Ajax.
• Similarly, some Web applications that use Ajax are built in a way that cannot be read by screen-reading technologies, such as JAWS. The WAI-ARIA standards provide a way to provide hints in such a case.
• Screen readers that are able to use Ajax may still not be able to properly read the dynamically generated content.
• The same-origin policy prevents some Ajax techniques from being used across domains, although the W3C has a draft of the XMLHttpRequest object that would enable this functionality. Methods exist to sidestep this security feature by using a special Cross Domain Communications channel embedded as an iframe within a page, or by the use of JSONP.
• The asynchronous callback-style of programming required can lead to complex code that is hard to maintain, to debug and to test.
• Because of the asynchronous nature of Ajax, each chunk of data that is sent or received by the client occurs in a connection established specifically for that event. This creates a requirement that for every action, the client must poll the server, instead of listening, which incurs significant overhead. This overhead leads to several times higher latency with Ajax than what can be achieved with a technology such as websockets.
• In pre-HTML5 browsers, pages dynamically created using successive Ajax requests did not automatically register themselves with the browser's history engine, so clicking the browser's "back" button may not have returned the browser to an earlier state of the Ajax-enabled page, but may have instead returned to the last full page visited before it. Such behavior — navigating between pages instead of navigating between page states — may be desirable, but if fine-grained tracking of page state is required, then a pre-HTML5 workaround was to use invisible iframes to trigger changes in the browser's history. A workaround implemented by Ajax techniques is to change the URL fragment identifier (the part of a URL after the "#") when an Ajax-enabled page is accessed and monitor it for changes. HTML5 provides an extensive API standard for working with the browser's history engine.
• Dynamic Web page updates also make it difficult to bookmark and return to a particular state of the application. Solutions to this problem exist, many of which again use the URL fragment identifier. On the other hand, as AJAX-intensive pages tend to function as applications rather than content, bookmarking interim states rarely makes sense. Nevertheless, the solution provided by HTML5 for the above problem also applies for this.
• Depending on the nature of the Ajax application, dynamic page updates may disrupt user interactions, particularly if the Internet connection is slow or unreliable. For example, editing a search field may trigger a query to the server for search completions, but the user may not know that a search completion popup is forthcoming, and if the Internet connection is slow, the popup list may show up at an inconvenient time, when the user has already proceeded to do something else.
• Excluding Google, most major Web crawlers do not execute JavaScript code, so in order to be indexed by Web search engines, a Web application must provide an alternative means of accessing the content that would normally be retrieved with Ajax. It has been suggested that a headless browser may be used to index content provided by Ajax-enabled websites, although Google is no longer recommending the Ajax crawling proposal they made in 2009.

Examples

JavaScript example

An example of a simple Ajax request using the GET method, written in JavaScript. 

get-ajax-data.js:

// This is the client-side script.

// Initialize the HTTP request.
var xhr = new XMLHttpRequest();
xhr.open('GET', 'send-ajax-data.php');

// Track the state changes of the request.
xhr.onreadystatechange = function () {
	var DONE = 4; // readyState 4 means the request is done.
	var OK = 200; // status 200 is a successful return.
	if (xhr.readyState === DONE) {
		if (xhr.status === OK) {
			console.log(xhr.responseText); // 'This is the output.'
		} else {
			console.log('Error: ' + xhr.status); // An error occurred during the request.
		}
	}
};

// Send the request to send-ajax-data.php
xhr.send(null);

send-ajax-data.php:

// This is the server-side script.

// Set the content type.
header('Content-Type: text/plain');

// Send the data back.
echo "This is the output.";
?>

Many developers dislike the syntax used in the XMLHttpRequest object, so some of the following workarounds have been created.

jQuery example

The popular JavaScript library jQuery has implemented abstractions which enable developers to use Ajax more conveniently. Although it still uses XMLHttpRequest behind the scenes, the following is the same example as above using the 'ajax' method. 

$.ajax({
	type: 'GET',
	url: 'send-ajax-data.php',
	dataType: "JSON", // data type expected from server
	success: function (data) {
		console.log(data);
	},
	error: function() {
		console.log('Error: ' + data);
	}
});

jQuery also implements a 'get' method which allows the same code to be written more concisely.

$.get('send-ajax-data.php').done(function(data) {
	console.log(data);
}).fail(function(data) {
	console.log('Error: ' + data);
});

Fetch example

Fetch is a new native JavaScript API. Although not yet supported by all browsers, it is gaining momentum as a more popular way to execute Ajax. According to Google Developers Documentation, "Fetch makes it easier to make web requests and handle responses than with the older XMLHttpRequest."

fetch('send-ajax-data.php')
    .then(res => res.text())
    .then(data => console.log(data))
    .catch(error => console.log('Error:', error));

// ES7 async/await example:

async function doAjax() {
    try {
        const res = await fetch('send-ajax-data.php');
        const data = await res.text();
        console.log(data);
    } catch (error) {
        console.log(error);
    }
}

doAjax();

As seen above, fetch relies on JavaScript promises.

HTML (updated)

From Wikipedia, the free encyclopedia

HTML
(Hypertext Markup Language)

The official logo of the latest version, HTML5.
Filename extension	.html .htm
Internet media type	text/html
Type code	TEXT
Developed by	W3C & WHATWG
Initial release	1993; 26 years ago
Latest release	5.2 (14 December 2017; 12 months ago)
Type of format	Document file format
Extended from	SGML
Extended to	XHTML
Standards	ISO/IEC 15445 W3C HTML latest recommendation HTML Living Standard
Open format?	Yes
Website	www.w3.org/html/ whatwg.org

Hypertext Markup Language (HTML) is the standard markup language for creating web pages and web applications. With Cascading Style Sheets (CSS) and JavaScript, it forms a triad of cornerstone technologies for the World Wide Web.

Web browsers receive HTML documents from a web server or from local storage and render the documents into multimedia web pages. HTML describes the structure of a web page semantically and originally included cues for the appearance of the document.

HTML elements are the building blocks of HTML pages. With HTML constructs, images and other objects such as interactive forms may be embedded into the rendered page. HTML provides a means to create structured documents by denoting structural semantics for text such as headings, paragraphs, lists, links, quotes and other items. HTML elements are delineated by tags, written using angle brackets. Tags such as <img /> and <input /> directly introduce content into the page. Other tags such as <p> surround and provide information about document text and may include other tags as sub-elements. Browsers do not display the HTML tags, but use them to interpret the content of the page.

HTML can embed programs written in a scripting language such as JavaScript, which affects the behavior and content of web pages. Inclusion of CSS defines the look and layout of content. The World Wide Web Consortium (W3C), maintainer of both the HTML and the CSS standards, has encouraged the use of CSS over explicit presentational HTML since 1997.

History

Development

Tim Berners-Lee in April 2009

 

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. 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 from 1990 he listed "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. 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.

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. 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.

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). 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.

HTML versions timeline

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)

January 14, 1997

HTML 3.2 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.

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

 

December 18, 1997

HTML 4.0 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", 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.

 

April 24, 1998

HTML 4.0 was reissued with minor edits without incrementing the version number.

 

December 24, 1999

HTML 4.01 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 ("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).

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.

 

October 28, 2014

HTML5 was published as a W3C Recommendation.

 

November 1, 2016

HTML 5.1 was published as a W3C Recommendation.

 

December 14, 2017

HTML 5.2 was published as a W3C Recommendation.

HTML draft version timeline

October 1991

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

 

June 1992

First informal draft of the HTML DTD, with seven 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.

 

June 1993

Hypertext Markup Language 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 one month later, followed by six further drafts published by IETF itself that finally led to HTML 2.0 in RFC 1866.

 

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 May 1994.

 

April 1995 (authored March 1995)

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

W3C began development of its own Arena browser as a test bed for HTML 3 and Cascading Style Sheets, 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. 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. 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." 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."

Logo of HTML5

January 2008

HTML5 was published as a Working Draft by the W3C.

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.

 

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. In January 2011, the WHATWG renamed its "HTML5" living standard to "HTML". The W3C nevertheless continues its project to release HTML5.

 

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.

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

 

2014 HTML5 – Proposed Recommendation and Recommendation

In September 2014, W3C moved HTML5 to Proposed Recommendation.

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

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, 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 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.
• XHTML 2.0 was a working draft, work on it was abandoned in 2009 in favor of work on HTML5 and XHTML5. 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.

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: 

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

The Document Type Declaration is for HTML5. If a declaration is not included, various browsers will revert to "quirks mode" for rendering.

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>.

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 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.

Some elements, such as the line break <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>
</head>

Headings: HTML headings are defined with the <h1> to <h6> tags: 

<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>

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. Note also that 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" /> 

<input type="file" /> 

<input type="checkbox" /> 

Comments:

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).

 

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

For example, <b>boldface</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</b> and <i>italic</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). 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 img element.

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.

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) 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.

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:

An example of an HTML 4 doctype

"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.

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, 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.

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. 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.

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.

Good semantic HTML also improves the accessibility of web documents. 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. 

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. XHTML 1.1 also states that XHTML 1.1 documents should be labeled with either MIME type.

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.

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. 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 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 />.

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: ).
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.

HTML5 variants

WHATWG HTML versus HTML5

The WHATWG considers their work as living standard HTML for what constitutes the state of the art in major browser implementations by Apple (Safari), Microsoft (Edge), Google (Chrome), Mozilla (Firefox), Opera (Opera), and others. HTML5 is specified by the HTML Working Group of the W3C following the W3C process. As of 2013, both specifications are similar and mostly derived from each other, i.e., the work on HTML5 started with an older WHATWG draft, and later the WHATWG living standard was based on HTML5 drafts in 2011.

Hypertext features not in HTML

HTML lacks some of the features found in earlier hypertext systems, such as source tracking, fat links and others. 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. 

Sometimes web developers or browser manufacturers remedy these shortcomings. For instance, wikis and content management systems allow surfers to edit the Web pages they visit.

WYSIWYG editors

There are some WYSIWYG editors (What You See Is What You Get), in which the user lays out everything as it is to appear in the HTML document using a graphical user interface (GUI), often similar to word processors. The editor renders the document rather than show the code, so authors do not require extensive knowledge of HTML.

The WYSIWYG editing model has been criticized, primarily because of the low quality of the generated code; there are voices advocating a change to the WYSIWYM model (What You See Is What You Mean). 

WYSIWYG editors remain a controversial topic because of their perceived flaws such as:

• Relying mainly on layout as opposed to meaning, often using markup that does not convey the intended meaning but simply copies the layout.
• Often producing extremely verbose and redundant code that fails to make use of the cascading nature of HTML and CSS.
• Often producing ungrammatical markup, called tag soup or semantically incorrect markup (such as <em> for italics).
• As a great deal of the information in HTML documents is not in the layout, the model has been criticized for its "what you see is all you get"-nature.