XML Formatter and Validator

Well-formed and valid are different words

XML has two distinct quality bars and they are routinely confused. Well-formed means the syntax is correct: every element is closed, tags nest properly rather than overlapping, attribute values are quoted, and there is exactly one root element. Valid means the document additionally conforms to a schema, such as an XSD or DTD, that specifies which elements may appear where.

This tool checks well-formedness, using your browser's own XML parser. That is the useful check for the everyday case of a configuration file or an API response that will not load, because malformed syntax is what breaks parsers. Schema validation requires the schema itself and is a separate step.

What the formatter preserves

• Attributes stay on their element in original order, with values re-quoted using double quotes.
• CDATA sections are preserved verbatim. Their contents are deliberately exempt from XML parsing, so reformatting them would change meaning.
• Comments are kept.
• The XML declaration, if present, is retained at the top.
• Elements with a single text child are kept on one line, which is almost always more readable than splitting three lines for one value.
• Empty elements are collapsed to self-closing form.

Whitespace is not always insignificant

This is the one thing to be careful about. In XML, whitespace inside an element is part of the content unless a schema says otherwise. Pretty-printing adds newlines and indentation between elements, and for the overwhelming majority of documents, where elements contain either child elements or a simple value, that is harmless.

It is not harmless for mixed content, meaning elements that contain both text and child elements interleaved, as in marked-up prose. Reformatting such a document can alter the rendered spacing. If your document contains mixed content and whitespace matters, use the minify direction or leave it alone.

Common parse failures

• Unescaped ampersands. A bare & in text must be written &. This is the most frequent XML error by a wide margin, and it appears constantly in URLs embedded in documents.
• Unescaped angle brackets. < in text must be <.
• Multiple root elements. A document may contain exactly one. Log files that append records without a wrapper produce this error.
• Mismatched case. XML tags are case sensitive, so <Item> is not closed by </item>.
• Undeclared namespace prefixes. Using soap:Body without declaring the prefix is not well-formed.

Questions