This is the oldest form of format for XML is DTD. The full form for DTD
is document type definition and it is inherited from SGML. It can be
said that inclusion of DTD has increased the popularity of XML but DTD
has its share of limitations.
1) DTD does not have features to support new inclusions such as namespaces.
2) Certain aspects of XML do not correlate with DTD.
3) Due to its inheritance from SGML it uses a non XML syntax.