XML Validator

XML validation is the process of checking an XML document to ensure it is syntactically correct (well-formed) and optionally conforms to a defined structure (valid). A well-formed XML document must follow strict XML syntax rules: every opening tag must have a matching closing tag, elements must be properly nested, attribute values must be quoted, and the document must have exactly one root element. ToolZeno's XML Validator performs comprehensive well-formedness checks plus additional quality checks for common issues like missing XML declarations, duplicate attributes, and non-standard encodings.

Well-formed XML follows the basic XML syntax rules defined by the W3C XML specification — proper tag matching, nesting, quoting, and a single root element. Any XML parser will reject a document that is not well-formed. Valid XML is additionally checked against a specific schema (DTD or XSD) that defines the allowed elements, attributes, and document structure for a specific application. ToolZeno's validator checks well-formedness plus additional quality rules. Schema (XSD/DTD) validation will be added as a future feature.

Yes. ToolZeno's XML Validator processes all XML entirely in your browser using the native DOMParser API. Your XML is never transmitted to any external server or stored anywhere. This makes the tool completely safe for validating confidential data including SOAP payloads, internal configurations, API responses, and sensitive business documents.

The most common XML errors include: (1) Unclosed tags — a tag that is opened but never closed; (2) Mismatched tags — an opening and closing tag with different names; (3) Illegal characters in tag names — tags cannot start with a digit or contain spaces; (4) Unescaped special characters — & < > " ' must be escaped as &amp; &lt; &gt; &quot; &apos; or wrapped in a CDATA section; (5) Multiple root elements — XML must have exactly one root element; (6) Duplicate attributes — the same attribute name appears twice on the same element; (7) Missing XML declaration — while optional, it's strongly recommended.

Each error in ToolZeno's validation report includes a 'Suggested Fix' that you can expand by clicking the error row. General XML fixing tips: (1) Use an XML formatter first — formatting reveals structural errors immediately; (2) Fix errors from top to bottom — a missing closing tag on line 5 may cause cascading errors on subsequent lines; (3) Check for unclosed tags — search for tags that were opened but never closed; (4) Escape special characters — replace &, <, >, ", ' with their XML entities; (5) Ensure the document has a single root element; (6) Remove or fix duplicate attribute names on elements.

Yes. ToolZeno's XML Validator checks that namespace prefix names are syntactically valid — they must start with a letter or underscore and contain only letters, digits, hyphens, underscores, or periods. The validator also flags common namespace issues as warnings. Full namespace URI resolution and namespace-aware schema validation will be added in a future release.

Yes. CDATA sections (<![CDATA[...]]>) and XML comments (<!-- ... -->) are fully supported. The validator correctly parses documents containing these constructs. CDATA sections allow you to embed characters like &, <, and > in XML without entity escaping. Comments are counted in the document statistics panel.

You can validate XML files up to 10MB directly in the browser. The validator uses the native DOMParser which is optimized for browser environments. For files larger than 5MB, performance depends on your device's CPU and available memory. For very large XML documents (>10MB), consider using a command-line XML validator such as xmllint on your local machine.