Well-formed and Valid XML Documents

Describing a document as being well-formed means that it conforms to basic rules of XML as set forth in the W3C XML 1.0 specification.

• It must have start and end tags for every element
• It must have one, and only one, document element
• Empty elements must be formatted correctly
• Elements must nest correctly
• Attribute values must always be in quotes

XML documents can be very simple, with nested markup created by the author and no Document Type Definition (DTD):

<?xml version="1.0" standalone="yes"?><conversation>   <greeting>Hello, world!</greeting>   <response>Stop the planet, I want to get off!</response></conversation>

A valid XML document is well-formed and has also been validated against a DTD or other XML schema. This means that an XML parser has determined that the document conforms to the rules of the schema associated with the document.

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