Well-Formed and Valid Documents
Not all XML documents are the same. In particular, two specific descriptions can be applied to XML documents to describe the content contained within them. XML docu-ments can be well formed, and they can also be valid. Validity implies “well-formed-ness,” but not vice versa. That’s because a valid XML document is a more strict form of a well-formed XML document. It’s like saying that a square is a rectangle, but not vice versa.
An XML document is well formed if it follows all the preceding syntax rules of XML. On the other hand, if it includes inappropriate markup or characters that cannot be processed by XML parsers, the document cannot be considered well formed. It goes without saying that an XML document can’t be partially well formed. And, by definition, if a document is not well formed, it is not XML. This means that there is no such thing as an XML document that is not well formed, and XML processors are not required to process these documents.
Although the property of “well-formedness” is a matter of making sure the XML docu-ment complies to syntactical rules, the property of validity is a different ballgame. A well-formed XML document is considered valid only if it contains a proper Document Type Declaration and if the document obeys the constraints of that declaration. In most cases, the constraints of the declaration will be expressed as a DTD or an XML Schema. Well-formed XML documents are designed for use without any constraints, whereas valid XML documents explicitly require these constraint mechanisms. In addition to con-straining the possible elements and the ordering of those elements in a document, valid XML documents can take advantage of certain advanced features of XML that are not available to merely well-formed documents due to their lack of a DTD or XML Schema. Some of these advanced features include linking mechanisms, value and range bounding, and data typing.
Although the creation of well-formed XML is a simple process, the use of valid XML documents can greatly improve the quality of document processes. Valid XML docu-ments allow users to take advantage of content management, business-to-business trans-actions, enterprise integration, and other processes that require the exchange of constrained XML documents. After all, any document can be well formed, but only spe-cific documents are valid when applied against a constraining DTD or schema.