Document Type Definitions
Before you can create an XML document, you need to define the rules of your
document: which elements (tags) can exist, which elements contain other elements,
which elements are optional or required, what attributes each element has, and so on.
You define the rules of an XML document in a Document Type Definition, or DTD. Every
markup language has a DTD so that parsers know how to verify the structure of
documents. Without a DTD, it is impossible for the parser to validate an XML document.
Every XML document requires a DTD.
If you are working with a predefined language, such as HTML or the Final Cut Pro XML
Interchange Format, the DTD has already been created for you. All you need to do is
follow the rules of the DTD to create valid Final Cut Pro XML.