Well-formed XML document

A well-formed document in XML is a document that adheres to the syntax rules specified by the XML 1.0 specification in that it must satisfy both physical and logical structures."

At its base level well-formed documents require that:

To be a well-formed document, rules must be established about the declaration and treatment of entities. Tags are case sensitive, with attributes delimited with quotation marks. Empty elements have rules established. Overlapping tags invalidate a document. Ideally, a well-formed document conforms to the design goals of XML. Other key syntax rules provided in the specification include:

A valid XML document is defined in the XML specification as a well-formed XML document which also conforms to the rules of a Document Type Definition (DTD). According to JavaCommerce.com XML tutorial, "Well formed XML documents simply markup pages with descriptive tags. You don't need to describe or explain what these tags mean. In other words a well formed XML document does not need a DTD, but it must conform to the XML syntax rules. If all tags in a document are correctly formed and follow XML guidelines, then a document is considered as well formed."

An XML processor that encounters a violation of the well-formedness rules is required to report such errors and to cease normal processing. This policy, occasionally referred to as draconian, stands in notable contrast to the behavior of programs that process HTML, which are designed to produce a reasonable result even in the presence of severe markup errors in the spirit of Postel's law ("Be conservative in what you send; be liberal in what you accept").

The concept of a well-formed document allows for a better understanding of the fundamental construction of XML. It helps to clarify XML beyond the typical sense of it. For example, while most XML Document Type Definitions utilize left and right angle brackets as content delimiters, strictly speaking this is not a necessity (though a delimiter should be terse and concise). The left and right angle bracket codes are a convention, albeit clear and distinctive, not an absolute requirement.

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