XML that meets the well-formedness constraints specified by the W3C.
Well-formed XML documents have element declarations within the document. All XML documents must be well-formed, but do not necessarily have to be valid.
An Extensible Markup Language (XML) document in which there is only one root element, and all of the elements nest properly within each other and are syntactically correct.
XML that follows the XML tag rules listed in the W3C Recommendation for XML 1.0, but doesn't have a DTD or schema. A well-formed XML document contains one or more elements; it has a single document element, with any other elements properly nested under it; and each of the parsed entities referenced directly or indirectly within the document is well formed. Well-formed XML documents are easy to create because they don't require the additional work of creating a DTD. Well-formed XML can save download time because the client does not need to download the DTD, and it can save processing time because the XML parser doesn't need to process the DTD.
XML with accurate beginning and ending tags, and containment relationships; that is, the XML conforms to the Worldwide Web Consortium (W3C) XML specification.