What is the difference between a well-formed XML and a valid XML?

Well-formed XML is a document that adheres to the basic syntactic rules of XML: each opening tag has a corresponding closing tag, elements are properly nested, attributes are correctly formatted, a valid encoding is used, etc. Such a document can be correctly parsed by any XML parser.

Valid XML is a well-formed document that additionally conforms to a specific schema (DTD, XSD, or other). Validity checks not only syntax but also structure, data types, the presence of mandatory elements, and their order according to the schema.

In summary:

• Well-formed XML guarantees correct syntax.
• Valid XML guarantees conformity to certain rules of structure and content.

Example: a document can be well-formed but not valid if a mandatory element defined in the schema is missing.