- XML documents must have a root element
- XML elements must have a closing tag
- XML tags are case sensitive
- XML elements must be properly nested
- XML attribute values must always be quoted
This should not be confused with a valid XML document, which is defined as a "well-formed" XML document which also conforms to the rules of a Document Type Definition (DTD) or an XML Schema (XSD), which W3C supports as an alternate to DTD.