XML in a Nutshell, 3rd Edition

Validation

A valid document includes a document type declaration that identifies the DTD that the document satisfies.^[1] The DTD lists all the elements, attributes, and entities the document uses and the contexts in which it uses them. The DTD may list items the document does not use as well. Validity operates on the principle that everything not permitted is forbidden. Everything in the document must match a declaration in the DTD. If a document has a document type declaration and the document satisfies the DTD that the document type declaration indicates, then the document is said to be valid. If it does not, it is said to be invalid.

There are many things the DTD does not say. In particular, it does not say the following:

• What the root element of the document is

• How many of instances of each kind of element appear in the document

• What the character data inside the elements looks like

• The semantic meaning of an element; for instance, whether it contains a date or a person’s name

DTDs allow you to place some constraints on the form an XML document takes, but there can be quite a bit of flexibility within those limits. A DTD never says anything about the length, structure, meaning, allowed values, or other aspects of the text content of an element or attribute.

Validity is optional. A parser reading an XML document may or may not check for validity. If it does check for validity, the program receiving data from the parser may or may not care about validity errors. In some cases, such ...