Markup languages such as HTML or SVG provide different ways to include or link programming scriptings to the document. In order to use scripting capabilities on the Web, it is recommended to read the Web Content Accessibility Guidelines [WCAG] before using scripts in your documents, in particular the guideline 6.

Specifications of existing scripting languages such as ECMAScript [ECMAScript] (an industry-standard scripting language based on JavaScript and JScript) only give the syntax, the grammar and the basic types' functionalities (String, Object, Number, ...). These specifications don't give the user access to the user agent functionalities (menus, scrollbars, user events, ...) or to the document (text contents, markup structure, style sheets, ...). This is where the Document Object Model stands in the context of scripting.

In November 1996, the W3C released the first Working Draft of the Extensible Markup Language (XML) specification. Many applications of XML were done since then: editors, repositories, databases, B2B systems, Web content, network protocols, etc.

The XML specification only provides the syntax and the grammar for XML. Being able to access the XML content in an application requires more than that: it requires an Application Programming Interface (API).

Generic APIs for XML can be mostly classified in two categories: event-based or tree-based. The DOM is a generic tree-based API for XML. For more information about event-based APIs and a comparison with tree-based APIs, read the [SAX] documentation "[What is an Event-Based Interface?]" or the DOM Frequently asked question (FAQ) "[What is the relationship between the DOM and SAX?]".

// get the HTML body element from the document
	  var childs = doc.getElementsByTagName("body");
	  // there is one HTML body element for each HTML 4.0 document
	  var body   = childs.item(0);
	  // change the value of the CSS2 color property
	  body.style.color = "black";

For a more advanced example, check the DOM Level 2 demo used in the main DOM page [Document Object Model].

The Core platform relies on an internal tree-like representation of the document, and enables you to traverse the hierarchy accordingly. The standard model of viewing a document is as a hierarchy of elements, with the computer building up an internal model of the document based on a kind of tree structure.

The DOM Range and Traversal modules can be used on the Core platform for tree manipulations and navigations.

The XML module extends the Core platform for specific XML 1.0 needs such as processing instructions, CDATA, and entities.

The DOM module provides a set of convenient easy-to-use ways to manipulate HTML documents. The initial HTML DOM merely describes methods, for example, for accessing an identifier by name, or a particular link. The HTML DOM is sometimes referred to as DOM Level 0 but has been imported into DOM Level 1.

This module includes XML-tree manipulation oriented events with tree mutation or user-oriented (such as mouse or HTML-specific) events.

The CSS module provides a set of convenient easy-to-use ways to manipulate CSS style sheets or styled documents.

Following the publication as a W3C Recommendation of the XML Information Set, the Core module is being aligned with this model. Node.baseURI now returns the XML Base as defined in the infoset. Facilities to manipulate the tree as an infoset have been added as well.

DOM Level 1 introduced a normalize method to merge adjacent text nodes. The Level 3 extension bring the ability to normalize a document given a set of parameters: "entities" (expand all entity references), "cdata-sections" (remove all CDATAsection nodes), "infoset" (transform the tree to match the XML Information Set model), "validate", etc.

Other user convenient methods have been introduced as well: Node.textContent, Text.wholeText, Node.lookupPrefix, Node.setUserData, etc.

A bootstrapping mechanism is now available when writing Java or ECMAScript code. The DOMImplementationRegistry contains the DOM implementations and can be query using a set of features. "XML Load-Save 3.0" will return the implementation that can support the XML module (any level) and the Load and Save Level 3 module.

The Level 3 Events module includes accessibility improvements, keyboard events, or event grouping. Namespaces are now used to refer to event types.

This module includes parameters to control the load and the save operations of a DOM tree: "entities", "comments", "namespaces", "infoset", "normalize-character". The set of parameters is shared by the extended normalization methods introduced in the Core Level 3 module. It also provides extended filtering mechanisms. As for SAX, the source of the document is defined by the user using an input source, the DOMInputSource.

This module provides user convenient methods for schema-driven manipulation of the tree. Methods such as canSetAttribute, canInsertBefore, etc. are available.

The XPath module provides a set of convenient easy-to-use functions to query a DOM tree using an XPath 1.0 expression.

The DOM requirements http://www.w3.org/TR/2001/WD-DOM-Requirements-20010419/ contains all requirements for the Levels of DOM. It is regularly updated to reflect the requirements of the latest Level.

Functionality equivalent to that exposed in Netscape Navigator 3.0 and Microsoft Internet Explorer 3.0 is referred to as "Level 0". There is no W3C specification for this Level.

The first Level of the DOM specifications, DOM Level 1, was completed in October 1998. Level 1 provides support for XML 1.0 and HTML.

The second Level of the DOM specifications, DOM Level 2, was completed in November 2000. Level 2 extends Level 1 with support for XML 1.0 with namespaces and adds supports for Cascading Style Sheets (CSS), events (user interface events and tree manipulation events), and enhances tree manipulations (tree ranges and traversal mechanisms).

The third Level of the DOM specifications, DOM Level 3, is under development. The Events, XPath, and Validation modules are in Last Call (as of October 15). Core and Load and Save are closed to move to Last Call as well.

In terms of implementations, early implementations are available in Xerces http://xml.apache.org/ and Mozilla http://www.mozilla.org/. Oracle and X-Hive are also implementing DOM Level 3 in their end-users softwares.