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An XML document is well-formed if it obeys the syntax rules of XML.

For example, in a well-formed XML document:

• Element and attribute names are legal XML names
• Markup characters < and & are escaped as entity references when used in text.
• Every element is closed
• Every attribute has a value
• Every attribute value is enclosed within matching quotes (double quote or apostrophe)
• Every element (except the root element) is a child of exactly one element.
• Comments are properly formed.

among many other rules.

Well-formedness is the minimum criteria for XML processors to read and interpret XML files.  Unlike with HTML, XML parsers are required to report errors of form.