What this does
Parses the document with your browser's XML parser and reports whether it is well-formed. A valid document shows its root element, element count, attribute count, and XML declaration.
What well-formed means
One root element, properly nested and closed tags, quoted attribute values, no duplicate attributes, declared namespace prefixes, only legal characters, and no references to undeclared entities. Error messages come from the parser and include the line and column; the wording differs a little between Chrome, Firefox, and Safari.
Schema validation
This tool checks well-formedness only. It does not validate against an XSD or DTD, because that needs a schema engine that cannot run reliably in the browser.
Privacy
Everything runs in your browser. Your input is never uploaded, stored, or put in the URL.