Q3. What is a valid XML document?
Ans. A valid XML document is not only a well-formed document but also one that conforms to the DTD rules. DTD rules help in defining the structure of XML documents with a list of valid elements. The following code snippet shows an example of a DTD document:
<!DOCTYPE email [
<!ELEMENT email (to, from, heading, body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
As you can see in the preceding code, we have defined DTD rules for the structure of an XML document. We have defined email as a parent element, which contains the four child elements: to, from, heading, and body. #PCDATA implies that all these elements will be parsed by the XML parser.