Well-formed XML documents are documents that are correct in their structure and syntax. They must conform to well-formedness constraints as outlined in the XML specification. These constraints require that elements, which are named content containers, properly nest within each other and use other markup syntax correctly. Well-formed XML elements are defined by their use, instead of by a rigid structural definition, allowing authors to create new elements.

For example, authors can describe documents in XML using elements of their own naming, such as ESSAY, SECTION, PARAGRAPH, NOTE and IMPORTANT. After writing the document, the author might change an instance of the PARAGRAPH element to DESCRIPTION to indicate an instance different from the other PARAGRAPH elements. This level of flexibility is indicative of well-formed XML and allows authors to describe documents as they see fit.