Question 1

What is XML validation?

Accepted Answer

XML syntax validation checks whether a document is well-formed: properly nested tags, single root element, closed tags, quoted attributes, and escaped special characters. Schema validation (DTD, XSD) additionally checks structure and data types — this tool validates well-formedness.

Question 2

What are the most common XML errors?

Accepted Answer

The five most common: (1) unclosed tags — every needs or self-close as , (2) multiple root elements — XML must have exactly one, (3) unescaped & — use & instead, (4) case-sensitive tag mismatch — ≠ , (5) attribute values without quotes — all attribute values must be quoted.

Question 3

Does well-formed XML guarantee valid XML against a schema?

Accepted Answer

Well-formed XML follows XML syntax rules. Valid XML is additionally checked against a schema (DTD or XSD) that defines allowed elements, attributes, and data types. This tool checks well-formedness. Schema validation requires a schema document and a validating parser.

Question 4

How do I fix 'undefined entity' errors?

Accepted Answer

If you're referencing named entities like   or &copy; that aren't defined in a DOCTYPE, the XML is invalid. Either define the entities in a DOCTYPE declaration, use numeric references (&#160; for non-breaking space), or switch to HTML if you need HTML entities.

Question 5

Can I validate XML against an XSD schema?

Accepted Answer

Schema validation requires a server-side validator or dedicated tool. This tool performs well-formedness validation using the browser's built-in XML parser. For XSD validation, use tools like xmllint (command line) or online schema validators.

Question 6

How do I validate XML syntax?

Accepted Answer

Paste your XML above for instant validation. The validator checks for well-formedness: properly nested tags, matching open/close elements, quoted attributes, and a single root element. XML must be well-formed before it can be processed. For formatting valid XML, use DevDecode's XML Formatter. For converting XML to JSON, use the XML to JSON Converter.

Question 7

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

Accepted Answer

Well-formed XML follows basic XML syntax rules: proper nesting, matched tags, quoted attributes, single root element. Valid XML additionally conforms to a Document Type Definition (DTD) or XML Schema (XSD) — defining allowed elements, attributes, and their structure. This validator checks well-formedness. Full schema validation requires your specific DTD/XSD.

XML Validator — Free Online XML Syntax Checker

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XML Validation: Well-Formedness and Common Errors

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