Limitations of DTD

DTDs have a number of obvious limitations. These are:

DTDs are written in non XML syntax.
DTDs do not support namespaces.
DTDs can only express the datatype of attributes in terms of explicit enumerations and a few coarse string formats; there is no facility for describing numbers, dates, currency values, and so forth. Furthermore, DTDs have no ability to express the datatype of character data in elements.

DTDs have a complex and fragile extension mechanism which is based on little more than string substitution. The XML schemas overcome these limitations and are much more expressive than DTDs. The additional expressiveness allows web applications to exchange XML data much more robustly without relying on ad hoc validation tools. Although XML schema may replace DTDs in future, DTDs still have a number of advantages. These are:

1. Widespread tools support: All SGML tools and many XML tools can process DTDs.
2. Widespread deployment: A large number of document types are already defined using DTDs. These include HTML, XHTML, DocBook, TEI, J2008, CALS, etc.
3. Widespread expertise and many years of practical application : Warts and all, DTDs are well understood by a large community of SGML and XMI programmers and consultants.
XML Schema offers a range of new features, which are:

Schema draft defines booleans, numbers, dates and times, URls, integers, decimal numbers, real numbers, intervals of time, SOL and Java primitive datatypes, etc. In addition to these simple, predefined types, there are facilities for creating other types and aggregate types.

Schema allows user defined types, called archetypes in the draft. An archetype allows you to define your own named datatype. For example a "Library book" datatype can be defined and then further defined as two elements, "Technical Book" and " Non Technical Book" to be of that type.

Schema allows attribute grouping. It is not uncommon to have several attributes that "go together", implying, common attributes that apply to all elements or several attributes that augment graphic or table elements. Attribute grouping allows the schema author to make this relationship explicit. In DTDs, the grouping can be achieved with a parameter entity, thus simplifying the process of authoring a DTD, but the information is not passed on to the processor.

Scheme facilitates namespace support. Since the introduction of Namespaces in XML, validation has become much more difficult. In fact, until the XML schema work is completed, it is just not practical to validate documents that use namespaces.

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