Document Object Model

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Hierarchy of objects in an example HTML DOM - Document Object Model
Hierarchy of objects in an example HTML DOM - Document Object Model

The Document Object Model (DOM) is a platform- and language-independent standard object model for representing HTML or XML and related formats.

A web browser is not obliged to use 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 its containing HTML page and browser state.

Because the 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). Hence the DOM is likely to be best suited for applications where the document must be accessed repeatedly or out of sequence order. If the application is strictly sequential and one-pass, the SAX model is likely to be faster and use less memory. In addition, non-extractive XML parsing models, such as VTD-XML, provide a new memory-efficient option.

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[edit] History

The World Wide Web Consortium (W3C) developed the W3C Document Object Model[1] in response to the development of various proprietary models for HTML, particularly those used in Web browsers. The existing vendor-specific interfaces were dubbed intermediate DOMs.

W3C began development of the DOM in the mid-1990s. Although the W3C never produced a specification for DOM 0, it was nonetheless a partially documented model and was included in the specification of HTML 4. By October 1998, the first specification of DOM (DOM 1) was released. DOM 2 was issued in November 2000, with specifics on the style sheet object model and style information manipulation. DOM 3 was released in April 2004 and is the current release of the DOM specification.

[edit] Levels

The W3C DOM specifications are divided into levels, each of which contains required and optional modules. To claim to support a level, an application must implement all the requirements of the claimed level and the levels below it. An application may also support vendor-specific extensions which don't conflict with the W3C standards. As of 2005, Level 1, Level 2, and some modules of Level 3 are W3C Recommendations which means they have reached their final form.

Level 0 
The application supports an intermediate DOM, which existed before the creation of DOM Level 1. Examples include the DHTML Object Model or the Netscape intermediate DOM. Level 0 is not a formal specification published by the W3C but rather a shorthand that refers to what existed before the standardization process.
Level 1 
Navigation of DOM (HTML and XML) document (tree structure) and content manipulation (includes adding elements). HTML-specific elements are included as well.
Level 2 
XML namespace support, filtered views and events.
Level 3 
Consists of 6 different specifications:
  1. DOM Level 3 Core;
  2. DOM Level 3 Load and Save;
  3. DOM Level 3 XPath;
  4. DOM Level 3 Views and Formatting;
  5. DOM Level 3 Requirements; and
  6. DOM Level 3 Validation, which further enhances the DOM

[edit] Specifications

[edit] Implementations

[edit] Web browsers

DOM Inspector inspecting Wikipedia's main page
DOM Inspector inspecting Wikipedia's main page

Earlier, when each Web browser exclusively supported its own intermediate DOM, interoperability problems were numerous. In order to be cross-browser compatible, that is, support multiple browsers, large parts of Dynamic HTML code had to be rewritten for each browser to be supported. A common DOM promised substantial simplification of the development of complex Web applications.

W3C DOM Level 1 has been a recommendation since 1 October 1998. The standardization effort did not bring forth an immediate change, because non-conformant browsers such as Internet Explorer 4.x and Netscape 4.x were still widely used in 2000. By 2005, large parts of W3C DOM were well-supported by common JavaScript-enabled Web browsers, including Microsoft Internet Explorer (version 5 (1999) and version 6 (2001)), Gecko-based browsers (like Mozilla and Firefox), Camino, Konqueror, Opera, and Safari. Web developers are starting to rely mostly or solely on W3C DOM, since it allows browser compatibility with a large audience [citation needed].

The article Comparison of layout engines (DOM) shows which methods and attributes may be used safely given certain browser requirements.

[edit] Other

[edit] See also

  • DOM Events
  • VTD-XML - A non-extractive XML processing model supporting random access and XPath
  • SAX - A set of APIs for accessing and manipulating XML documents in a sequential manner.
  • JDOM - A Java-based document object model for XML that integrates with DOM and SAX and uses parsers to build the document.
  • Comparison of layout engines (DOM)
  • Ajax - A methodology employing DOM in combination with techniques for retrieving data without reloading a page.
  • ECMAScript
  • TinyXml - efficient lightweight platform-independent XML library for C++

[edit] External links

[edit] References

  1. ^ W3C Document Object Model (DOM) (html). W3C Architecture domain. Retrieved on 2006-12-05.
v • d • e
Standards of the World Wide Web Consortium
Recommendations CSS · CGI · DOM · HTML · RDF · SVG · SOAP · SMIL · SRGS · SSML · VoiceXML · WSDL · XACML · XHTML · XML · XML Events · XForms · XML Information Set · XML Schema (W3C) · XPath · XPointer · XQuery · XSL Transformations · XLink · XML Base · SISR · OWL · MathML · RDF Schema · Timed Text · CDF
Notes XHTML+SMIL
Working Drafts CURIE · CCXML · InkML · XFrames · XFDL  · WICD  · XHTML+MathML+SVG
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