You can get the detailed information about HTML DOM from
http://www.w3schools.com/htmldom/default.asp
HTML DOM is a programming interface for HTML documents.
HTML DOM defines how you can access and change the content of HTML documents.
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What is HTML DOM?
The DOM stands for the Document Object Model.
The HTML DOM is the Document Object Model for HTML.
The HTML DOM defines a standard set of objects for HTML, and a standard way to access and manipulate HTML objects.
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Why HTML DOM?
Because we need an easy and standard way to access the content of HTML documents, which can be used by all types of browsers and all types of programming languages.
HTML DOM defines how to access HTML elements, and how to change, add, or delete their content, attributes and styles.
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The DOM Standard
The DOM Standard is developed by W3C (The World Wide Web Consortium):
"The W3C Document Object Model (DOM) is a platform and language neutral interface that allows programs and scripts to dynamically access and update the content, structure, and style of a document".
Key players in the development of the DOM has been people from ArborText, IBM, Inso EPS, JavaSoft, Microsoft, Netscape, Novell, the Object Management Group, SoftQuad, Sun Microsystems, and Texcel.