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20.4. Constraints

In addition to defining the basic structures used in documents and DTDs, XML 1.0 defines a list of rules regarding their usage. These constraints put limits on various aspects of XML usage, and documents cannot in fact be considered to be "XML" unless they meet all of the well-formedness constraints. Parsers are required to report violations of these constraints, though only well-formedness constraint violations require that processing of the document halt completely. Namespace constraints are defined in Namespaces in XML, not XML 1.0.

20.4.1. Well-Formedness Constraints

Well-formedness refers to an XML document's physical organization. Certain lexical rules must be obeyed before an XML parser can consider a document well-formed. These rules should not be confused with validity constraints, which determine whether a particular document is valid when parsed using the document structure rules contained in its DTD. The Backus-Naur Form (BNF) grammar rules must also be satisfied. The following sections contain all well-formedness constraints recognized by XML Version 1.0 parsers, including actual text from the 1.0 specification.

20.4.2. Validity Constraints

The following sections contain all validity constraints that are enforced by a validating parser. Each includes actual text from the XML 1.0 specification and a short explanation of what the constraint actually means.

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