What is DTD
DTD (document type definition)
The Document Type Definition (DTD) defines the permitted elements and associated attributes for an XML document. The document type declaration corresponds roughly to a vocabulary for a very specific class of XML documents and its grammar also represents the rules for determining whether the content of a document is valid or invalid. With the help of a Document Type Definition, it is possible to check a document for its validity, in that its structure corresponds to the DTD. A valid document is always well-formed.
The syntax of DTDs is only expressive enough to decide which elements can be the content of an XML document. However, document type declarations do not allow the content of the elements themselves to be viewed in a differentiated manner. A precise differentiation of data types has only been realized with the XML schema standardized by the World Wide Web Consortium (W3C).
If no document type definition or a schema is defined for a document, it cannot be checked for validity but only for well-formedness. In addition to DTDs and the W3C XML schema, Relax NG (Regular Language for XML, New Generation) is another way of describing the structure of an XML document.
In connection with XML documents, a distinction must be made between well-formedness and validity:
- Well-formedness refers to the syntactic correctness of an XML document.
- An XML document is valid if its structure corresponds to the associated document type declaration or, alternatively, to the referenced schema. Valid XML documents are always well-formed.
The element types permitted in the document with their content models and attributes can be defined in DTDs. A content model defines the permitted content of an element. The following content models are established by the XML specification:
- EMPTY, such an element has no content, but may have attributes.
- ANY, as long as it is well-formed XML, the element can have any content.
- #PCDATA, the element contains only character data
- Mixed content, elements can contain further sub-elements and character data.
- Element content, such an element contains only sub-elements.
- DTDs use a specific - not XML - syntax. This means that your own tools are required.
- Data typing is not supported.
- There is only the data type #PCDATA.
- Information on cardinality can only be used insufficiently.
- Lack of compatibility with XML namespaces
- Bad expandability options.
- The definitions in DTDs are generally global, which contradicts object-oriented modeling.
SVG Scalable Vector Graphics (SVG) is a graphic format for a language that is specified by a DTD and is used to represent two-dimensional graphics. SVG is used, for example, for the presentation of graphic content on cell phones or PDAs.
SMIL Synchronized Multimedia Integration Language (SMIL) is a language that was standardized in a first version by the W3C in 1998 and is used for the interactive representation of texts, videos or even images.
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