Document Object Model

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Document Object Model (DOM) is a description of how an HTML or XML document is represented in an object-oriented fashion. DOM provides an application programming interface to access and modify the content, structure and style of the document.

Various interfaces were initially implemented by web browsers to manipulate elements in an HTML document through JavaScript. This prompted the World Wide Web Consortium (W3C) to come up with a series of standard specifications that defined the W3C Document Object Model (W3C DOM). The W3C specifications are platform and language independent. The previous vendor-specific interfaces were dubbed intermediate DOMs.

Using DOM, the document is accessed in a tree form and this is also the data structure that most XML parsers (e.g., Xerces) and XSL processors (e.g., Xalan) have been developed to make use of. Such an implementation requires that the entire content of a document be parsed and stored in memory. Hence, DOM is best used for applications where the document elements have to be accessed and manipulated in an unpredictable sequence and repeatedly. If the application involves a sequential or one-time selective read or write per processed document, DOM presents a considerable overhead. The sequential SAX model is advantageous in such a case in terms of speed and memory consumption.

1 Levels
2 Use in web browsers
3 Specifications
4 See also
5 External links

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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 underlying levels. An application may also support vendor-specific extensions if they don't conflict with the W3C standards. As of 2005, Level 1 and 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, which is the intermediate DOM developed by Microsoft, and the unnamed 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 to manipulate content (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 enhance the DOM.

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Use in web browsers

Image:DOM Inspector.png Since each web browser used to exclusively support its own intermediate DOM, inter-operability problems were abundant. In order to be cross-browser compatible, i.e. support multiple browsers, large parts of Dynamic HTML code had to be rewritten for each browser to be supported. A common DOM promised to greatly simplify 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, since non-conformant browsers such as Internet Explorer 4x and Netscape 4.x were still widely used in 2000. As of 2005, large parts of W3C DOM are well supported by common JavaScript-enabled web browsers, including Microsoft Internet Explorer (version 5 (1999), ... version 6 (2001)), Gecko-based browsers (like Mozilla and Firefox) and Opera. Web developers are starting to mostly or solely rely on W3C DOM, since it doesn't preclude from reaching a large audience.

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

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