What is document standards?

XML is a general-purpose specification for creating custom markup languages. XML itself does not help your documentation needs. You need an document structure.

For documentation purpose we use Document Type Defenition (DTD) or XML Schemas. A DTD is a generic document which describes the content of another document. In other words, it defines the elements and attributes which constitute the markup which i.a. a browser will interpret to display and layout the document. A Document validation is a process through which the conformity of a document is checked against its description (DTD).

An XML document that complies with a particular schema/DTD is said to be valid.