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XML: A Look At Some Real Data Part 2

But one of the problems when creating arbitrary tags is how to explain the relationship between elements. In HTML, this is explicitly defined by its basic object model. For instance, HTML has a hierarchy of objects that all HTML documents share (even if they aren't explicitly stated in the HTML markup). That structure looks basically like this:
 <HTML> <HEAD> <TITLE>Some Title</TITLE> </HEAD> <BODY> document contents </BODY> </HTML> 
The top-level HTML declaration contains the entire document and defines it as a Web page. The HTML element contains header information, such as the TITLE, that is itself contained within the HEAD element. The visible portion of the document is contained in the BODY element. The relationship between these elements is described in a template that browsers use called a Document Type Definition (DTD). XML vocabularies can also have DTDs and the CDF vocabulary used in this sample application has one that describes the relationship between CHANNELs, AUTHORs, ABSTRACTs and ITEMs. It also describes whether these elements have attributes, such as the HREF property of CHANNELs and ITEMs that allow those to point to specific Web pages. But XML, unlike SGML, doesn't require that DTDs be used to describe the relationships between elements. Instead, the hierarchy of arbitrarily-defined objects in an XML document are implied by their position in the hierarchy.


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