What is DTD for an XML document

A DTD is the Document Type Definition for an XML document. It defines the way in which the elements in an XML document should look like. The type of data that should be present in the nodes, the number of child nodes that should be present, the attributes that should be used for an element are all defined in a DTD file.

Based on this the XML document is validated. If an XML document is created based on this rule then it is said to be a valid document. All XML documents are validated against a DTD or a schema.

A schema is an advanced version of a DTD. Schemas are used now-a-days instead of DTD as they have more flexibility in validating the XML documents.

Signs like '+', '*', and '?' are used to define the number of child nodes that should be present within an element. The elements are defined using the ELEMENT keyword. The attributes are defined using the ATTLIST keyword. The type of data is either CDATA or PCDATA.

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