Well-formed vs Valid XML
Well-formed means that a textual object meets the W3C requirements for being XML.
Valid means that well-formed XML meets additional requirements given by a specified schema.
Per the W3C Recommendation for XML:
[Definition: A data object is an XML document if it is
well-formed, as defined in this specification. In addition, the
XML document is valid if it meets certain further constraints.]
- A document that is not well-formed is not XML. (Well-formed XML is commonly used but technically redundant.)
- Being valid implies being well-formed.
- Being well-formed does not imply being valid.
- Although the W3C Recommendation for XML defines validity to be against a DTD, conventional use allows the term to be applied for conformance to XML schemas specified via XSD, RELAX NG, Schematron, or other methods.
Examples of what causes a document to be…
- An element lacks a closing tag (and is not self-closing).
- Elements overlap without proper nesting:
- An attribute value is missing a closing quote that matches the
&are used in content rather than
- Multiple root elements exist.
- Multiple XML declarations exist, or an XML declaration appears other than at the top of the document.
- An element or attribute is missing but required by the XML schema.
- An element or attribute is used but undefined by the XML schema.
- The content of an element does not match the content specified by the XML schema.
- The value of an attribute does not match the type specified by the XML schema.
Technically, colon characters are permitted in component names in XML. However, colons should only be used in names for namespace purposes:
The Namespaces in XML Recommendation [XML Names] assigns a
meaning to names containing colon characters. Therefore, authors
should not use the colon in XML names except for namespace purposes,
but XML processors must accept the colon as a name character.
Therefore, another term, namespace-well-formed, is defined in the Namespaces in XML 1.0 W3C Recommendation that implies all of the XML rules for well-formedness plus those governing namespaces and namespace prefixes.
Colloquially, the term well-formed is often used where namespace-well-formed would be more precise. However, this is a minor technical manner of less practical consequence than the distinction between well-formed vs valid XML described in this answer.