More on the XML recommendation

Document Type Definitions

A Document Type Definition (DTD) is a way to constraint XML documents of a specific class.

XML is really a meta-language: through the use of DTD's it allows designers to define syntax rules. Documents (a.k.a. instances) can be checked for its conformance to a DTD.

A DTD-less XML instance is said to be well-formed, an XML document that is conformant to a DTD is said to be valid.

The DTD for the wine document:

<!ELEMENT catalog    (class+)>
<!ELEMENT class      (category+)>
<!ATTLIST class      name (red | white | rose) #IMPLIED>
<!ELEMENT category   (item+)>
<!ATTLIST category   name CDATA #REQUIRED>
<!ELEMENT item (winery, type, year, rating, stock, price)>
<!ELEMENT winery     (#PCDATA)>
<!ELEMENT type       (#PCDATA)>
<!ELEMENT year       (#PCDATA)>
<!ELEMENT rating     (#PCDATA)>
<!ELEMENT stock      (#PCDATA)>
<!ELEMENT price      (#PCDATA)>
<!ATTLIST price      currency CDATA #REQUIRED>

What can you do with XML