Valid and well-formed XML documents
An XML document
must be valid, well-formed, or both.
Valid documents
To define a set of tags for use in a particular application,
XML uses a separate document named a document type definition (DTD).
A DTD states what tags are allowed in an XML document and defines
rules for how those tags can be used in relation to each other.
It defines the elements that are allowed in the language, the attributes
each element can have, and the type of information each element
can hold. Documents can be verified against a DTD to ensure that they
follow all the rules of the language. A document that satisfies
a DTD is said to be valid.
If a document uses a DTD, the DTD must immediately follow
the declaration.
XML Schema provides an alternative mechanism for describing
and validating XML data. It provides a richer set of datatypes than
a DTD, as well as support for namespaces, including the ability
to use prefixes in instance documents and accept unknown elements
and attributes from known or unknown namespaces. For more information,
see the W3C XML Schema page
.
Well-formed documents
The second way to specify XML syntax is to assume that a document
is using its language properly. XML provides a set of generic syntax
rules that must be satisfied, and as long as a document satisfies
these rules, it is said to be well–formed. All valid documents
must be well-formed.
Processing well-formed documents is faster than processing
valid documents because the parser does not have to verify against
the DTD or XML schema. When valid documents are transmitted, the
DTD or XML schema must also be transmitted if the receiver does
not already possess it. Well–formed documents can be sent
without other information.
XML documents should conform to a DTD or XML schema if they
are going to be used by more than one application. If they are not
valid, there is no way to guarantee that various applications will
be able to understand each other.