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The Document Object Model (DOM) is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents.[1] Aspects of the DOM (such as its "Elements") may be addressed and manipulated within the syntax of the programming language in use. The public interface of a DOM is specified in its application programming interface (API).
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The history of the Document Object Model is intertwined with the history of the "browser wars" of the late 1990s between Netscape Navigator and Microsoft Internet Explorer, as well as with that of JavaScript and JScript, the first scripting languages to be widely implemented in the layout engines of web browsers.
JavaScript was released by Netscape Communications in 1996 within Netscape Navigator 2.0. Netscape's competitor, Microsoft, released Internet Explorer 3.0 later the same year with a port of JavaScript called JScript. JavaScript and JScript let web developers create web pages with client-side interactivity. The limited facilities for detecting user-generated events and modifying the HTML document in the first generation of these languages eventually became known as "DOM Level 0" or "Legacy DOM". No independent standard was developed for DOM Level 0, but it was partly described in the specification of HTML4.
Legacy DOM was limited in the kinds of elements that could be accessed. Form, link and image elements could be referenced with a hierarchical name that began with the root document object. A hierarchical name could make use of either the names or the sequential index of the traversed elements. For example, a form input element could be accessed as either "document.formName.inputName" or "document.forms[0].elements[0]".
The Legacy DOM enabled client-side form validation and the popular "rollover" effect.
In 1997, Netscape and Microsoft released version 4.0 of Netscape Navigator and Internet Explorer, adding support for Dynamic HTML (DHTML), functionality enabling changes to a loaded HTML document. DHTML required extensions to the rudimentary document object that was available in the Legacy DOM implementations. Although the Legacy DOM implementations were largely compatible since JScript was based on JavaScript, the DHTML DOM extensions were developed in parallel by each browser maker and remained incompatible. These versions of the DOM became known as the "Intermediate DOM."
The Intermediate DOMs enabled the manipulation of Cascading Style Sheet (CSS) properties that influence the display of a document. They also provided access to a new feature called "layers" via the "document.layers" property (Netscape Navigator) and the "document.all" property (Internet Explorer). Because of the fundamental incompatibilities in the Intermediate DOMs, cross-browser development required special handling for each supported browser.
Subsequent versions of Netscape Navigator abandoned support for its Intermediate DOM. Internet Explorer continues to support its Intermediate DOM for backwards compatibility.
The World Wide Web Consortium (W3C), founded in 1994 to promote open standards for the World Wide Web, brought Netscape Communications and Microsoft together with other companies to develop a standard for browser scripting languages, called "ECMAScript". The first version of the standard was published in 1997. Subsequent releases of JavaScript and JScript would implement the ECMAScript standard for greater cross-browser compatibility.
After the release of ECMAScript, W3C began work on a standardized DOM. The initial DOM standard, known as "DOM Level 1," was recommended by W3C in late 1998. About the same time, Internet Explorer 5.0 shipped with limited support for DOM Level 1. DOM Level 1 provided a complete model for an entire HTML or XML document, including means to change any portion of the document. Non-conformant browsers such as Internet Explorer 4.x and Netscape 4.x were still widely used as late as 2000.
DOM Level 2 was published in late 2000. It introduced the "getElementById" function as well as an event model and support for XML namespaces and CSS. DOM Level 3, the current release of the DOM specification, published in April 2004, added support for XPath and keyboard event handling, as well as an interface for serializing documents as XML.
By 2005, large parts of W3C DOM were well-supported by common ECMAScript-enabled browsers, including Microsoft Internet Explorer version 6 (2001), Opera, Safari and Gecko-based browsers (like Mozilla, Firefox, SeaMonkey and Camino).
Web browsers are not obliged to use the DOM in order to render an HTML document. However, the DOM is required by JavaScript scripts that wish to inspect or modify a Web page dynamically. In other words, the Document Object Model is the way JavaScript sees the browser state and the HTML page it contains.
When an HTML page is rendered in a browser, the browser assembles all the elements (objects) that are contained in the HTML page, downloaded from the web-server in its memory. Once done the browser then renders these objects in the browser window as text, forms, input boxes, etc. Once the HTML page is rendered in the web-browser window, the browser can no longer recognize individual HTML elements (Objects).
Since the JavaScript enabled browser uses the Document Object Model (DOM), after the page has been rendered, JavaScript enabled browsers are capable of recognizing individual objects in an HTML page.
The HTML objects, which belong to the DOM, have a descending relationship with each other.
The topmost object in the DOM is the Navigator (i.e. Browser) itself. The next level in the DOM is the browser's Window, and under that are the Documents displayed in Browser's Window.
DOM
|-> Window
|-> Document
|-> Anchor
|-> Link
|-> Form
|-> Text-box
|-> Text Area
|-> Radio Button
|-> Check Box
|-> Select
|-> Button
Three levels or parts of DOM exist:[1]
Because DOM supports navigation in any direction (e.g., parent and previous sibling) and allows for arbitrary modifications, an implementation must at least buffer the document that has been read so far (or some parsed form of it).
Web browsers rely on layout engines to parse HTML into a DOM. Some layout engines such as Trident/MSHTML and Presto are associated primarily or exclusively with a particular browser such as Internet Explorer and Opera respectively. Others, such as WebKit and Gecko, are shared by a number of browsers, such as Safari, Google Chrome, RockMelt, Firefox or Flock. The different layout engines implement the DOM standards to varying degrees of compliance.
DOM implementations:
APIs that expose DOM implementations:
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