Well-Formed XML Documents

What does it mean for an XML document to be well-formed? To be well-formed, an XML document must follow the syntax rules set up for XML by W3C in the XML 1.0 specification (which you can find at http://www.w3.org/TR/REC-xml, and which we'll discuss in more detail in the next chapter). Informally, well-formedness often means that the document must contain one or more elements, and one element, the root element, must contain all the other elements. Each element also must nest inside any enclosing elements properly. For example, this document is not well-formed because the </GREETING> closing tag comes after the opening <MESSAGE> tag for the next element:

<?xml version="1.0" encoding="UTF-8"?>
<DOCUMENT>
    <GREETING>
        Hello From XML
    <MESSAGE>
    </GREETING>
        Welcome to the wild and woolly world of XML.
    </MESSAGE>
</DOCUMENT>