XML Validator online

The XML Validator checks your XML for well-formedness — meaning it has a single root element, all tags are properly closed and correctly nested, attributes are quoted, and special characters are escaped. Errors are reported with exact line and column numbers so you can jump straight to the problem. Related tools: XML Formatter, XML Minifier, JSON Validator, XML to JSON.

What Does the XML Validator Check?

Well-formedness is the foundational requirement for any XML document. The validator uses the browser's built-in XML parser (the same engine that powers every web browser's XML processing) to check for:

• Single root element — XML must have exactly one top-level element wrapping all content
• Closed tags — every opening tag must have a matching closing tag, or be self-closed (<br />)
• Correct nesting — tags must close in the reverse order they opened, no overlapping
• Quoted attributes — all attribute values must be wrapped in single or double quotes
• Escaped special characters — &, <, > in content must use entities or CDATA sections
• Valid XML declaration — the <?xml version="1.0"?> declaration must be correctly formed if present
• Unique attributes — the same attribute cannot appear twice on one element
• Namespace prefixes — any prefix used must be bound to a namespace URI

Well-Formed vs Valid XML — What's the Difference?

These terms are often confused. Well-formed means the document follows the core XML syntax rules above — it can be parsed by any XML parser without error. Valid means it additionally conforms to a specific schema, either a DTD (Document Type Definition) or an XSD (XML Schema Definition), which describes what elements and attributes are allowed and in what structure. This tool checks well-formedness, which must always pass before schema validation is even attempted. If your XML is not well-formed, no parser can process it regardless of schema.

Common XML Errors and How to Fix Them

• Unclosed tag — add the matching </tagname> or change to a self-closing <tagname />
• Multiple root elements — wrap all top-level elements inside a single container element
• Unquoted attribute value — add double or single quotes around the attribute: id="123"
• Bare ampersand — replace & with &, or wrap the content in a CDATA section
• Bare less-than sign — replace < in text content with <
• Wrong nesting — reorder closing tags so they mirror the opening order exactly
• Unknown entity — use the five predefined XML entities (& < > ' ") or define your own in a DTD