What is the difference between well-formed and valid XML document?

 Posted by Charugoel on 2/4/2009 | Category: Web Services, Remoting Interview questions | Views: 18187

W3C, in the XML specification, has defined certain rules that need to be followed while creating XML documents. The examples of such rules include: having exactly one root element, having end-tag for each start- tag, using single/double quotes for attribute values, and so on. If an XML document follows all these rules, it is said to be well-formed document and XML parsers can be used to parse and process such documents.
Document Type Definitions (DTDs) or XML Schemas can be used to define the structure and content of a specific class of XML documents. This includes the parent-child relationship details, attribute lists, data type information, value restrictions, etc. In addition to the well-formedness rules, if an XML document also follows the rules specified in the associated DTD/Schema, it is said to be a valid XML document. All valid XML documents are well-formed; but the reverse is not always true, that is, well-formed XML documents do not necessarily have to be valid.

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