`". ## Why the distinction matters The key relationship is that validity presupposes well-formedness but not the reverse. A document can be perfectly well-formed and still invalid against a schema, and most tools that "parse" XML only check well-formedness unless you explicitly ask them to validate. So when something rejects your XML, the first question is which bar it failed: a well-formedness error is a syntax mistake in the document itself, while a validity error means the syntax is fine but the content does not match the schema it was checked against. They are fixed in different places.

# Well-Formed vs Valid XML > Well-formedness is XML's baseline: one root, properly nested and matched tags, quoted attributes, and escaped specials. Validity is a stronger, separate claim that a document also follows a schema. A parser rejects ill-formed XML outright, which is why these rules come first. Source: https://ronutz.com/en/learn/xml-well-formedness Updated: 2026-07-01 Related tools: https://ronutz.com/en/tools/xml-decoder --- Two different words get used for "correct" XML, and they mean different things. Confusing them leads to a lot of wasted debugging. ## Well-formed **Well-formed** is the baseline every XML document must meet to be XML at all. The rules are syntactic: - There is exactly one **root element** containing everything else. - Every element has a matching end tag, or is self-closing. `` must be closed by ``. - Elements are **properly nested**: `` is fine, `` is not, because the inner element must close before the outer one. - **Attribute values are quoted**, with single or double quotes, and an element cannot repeat an attribute name. - The special characters `<` and `&` are **escaped** in text (as `<` and `&`), because bare they would start a tag or an entity. A document that breaks any of these is **not well-formed**, and a conforming XML parser must reject it rather than guess at the author's intent. This strictness is deliberate: it is the difference between XML and the forgiving error-recovery of HTML. ## Valid **Valid** is a stronger and entirely separate claim. A document is valid when, in addition to being well-formed, it conforms to a **schema**, a set of rules describing which elements are allowed, in what order, with which attributes and data types. Schemas come in several flavours: the older DTD, and the more expressive XML Schema (XSD) and RELAX NG. Validation is what catches "this document is syntactically fine but a `` is missing its required ``". ## Why the distinction matters The key relationship is that validity presupposes well-formedness but not the reverse. A document can be perfectly well-formed and still invalid against a schema, and most tools that "parse" XML only check well-formedness unless you explicitly ask them to validate. So when something rejects your XML, the first question is which bar it failed: a well-formedness error is a syntax mistake in the document itself, while a validity error means the syntax is fine but the content does not match the schema it was checked against. They are fixed in different places.