How are DTDs/XML Schemas important while building XML applications?
Document Type Definition (DTD) or XML Schema is used to define the structure and other constrains for the XML documents. If an XML document has an associated DTD/Schema, it is said to be a valid XML document, and it ensures that the XML document structure and data adheres to the predefined rules. While using the XML document in an application, once the XML document is validated, it is then not required to assert/check for parent-child relationship, presence/absence of elements/attributes, data-value range checks, etc. The schema validation already took care of all such issues. If a valid XML document is being used as the media of data transfer between two parties, both can be rest assured that the XML document is as expected. In addition, DTDs/Schemas have many other benefits, including help in understanding the resultant XML document structure/constraints (documentation), and defining DTDs/Schemas is also a good design step.