HTML, markup language of Web pages and interfaces

Defined for web pages, it is also a standard format for any documents displayable locally by a web browser or a recent word processor and now is also a language of user interface for web applications, and has replaced Flash.

HTML, the hypertext markup language is a subset of SGML, (invented by IBM in 1969) defined by the W3C consortium. It is a document description language, that uses tags for properties. This is the format recognized by web browsers.
DHTML, dynamic HTML, is the combination of HTML and JavaScript. The CSS, cascading style sheet, adds the style sheet feature of word processor to HTML.

The development has stagnated from version 4, the W3C polarizing on XHTML but could not reach a consensus on its definition.
An independent initiative to a new standard , the WHATWG, re-launched it under the name of HTML 5, which led the W3C to rally to this new format and to make it the current standard.

HTML 5 is no longer a document format but an interface language for online applications or even with new operating systems as Firefox OS, Chrome OS, webOS, for local applications too. The JavaScript section on this site gives several examples of using HTML as interface as local application with Node.js, even if the programming language is PHP.

Originally, the language is not expandable as is XML. The developer can not add new tags and assign them a role that browsers can consider. But HTML 5 frameworks implement the Web Components standard to give this extensibility, they are mainly Polymer, X-Tags from Mozilla and partially Angular. These interface components working on the client side can eventually interact with the server, through Ajax, WebSocket, WebRTC.

Features of HTML

Tag based, uses < > for delimiters.
Its syntax is derived from that of XML, but tags are not necessarily closed.
Unrecognized statements are ignored by the rendering engine (from the specification).
Unlimited embedding of constructs. A limited number of tags are not containers.
Unlike XML, tags have a predefined meaning that is standard and recognized by user agents (mainly browsers).

Sample code: The minimal code for displaying "Hello World":

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>Hello by Scriptol.com</title>
</head>
<body>
    <p>Hello World!</p>
</body>
</html>

Tools and documentation

W3C
Site of the consortium that defines the language. Specifications of all standards and manuals.
Validator
Check the validity of your code.
HTML 5 Forms
Code of all form objects to check if they are implemented in the browser.

There are many HTML editors working in WYSIWYG (direct editing in the final presentation) and which are well-designed. Here is a list:

See also: XML, another format issued from SGML, like HTML.