The Document Object Model is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. 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).
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.elements".
The Legacy DOM enabled client-side form validation and the popular "rollover" effect.
The Intermediate DOMs enabled the manipulation of Cascading Style Sheet (CSS) properties which 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.
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 Application Suite and Camino).
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