• WWW browsers are lenient about the quality of the HTML.
• The XML specification does not allow XML parsers to be lenient.
• A correct document is said to be well-formed.
• An XML document that is not well-formed must be reported as being in error.
• An XML document may use a document type definition (DTD).
• The DTD is a sequence of rules describing the markup language.
• Can appear in the same file as the XML document or in a separate file.
• XML that conforms to a DTD is said to be a valid document.
• Even if there is a DTD, not all parsers check that the XML conforms to the DTD.

<!ELEMENT consumables (product*)>
<!ELEMENT product (category, item, price)>
<!ELEMENT category (#PCDATA)>
<!ELEMENT item (#PCDATA)>
<!ELEMENT price (#PCDATA)>

http://www.dur.ac.uk/barry.cornelius/papers/xml.processing.using.php/code/consumables.dtd.