Document Type Definitions (DTDs)
XML is immensely flexible. You can have any elements you want in your XML documents; you can define what attributes each element has, what children an element has, and so on. This flexibility brings with it the potential for enormous power, but equally it brings with it the potential for disorder and untidiness.
When you remember that a primary purpose of XML technologies is to enable a computer to speak to a computer and exchange information seamlessly, you can probably see that any untidiness might result in a breakdown or an unreliable exchange of information.
To minimize the likelihood of a multi-tier XML application breaking down, it is wise to rein in the flexibility and apply some logical, practical constraints ...
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