XML Validator
Check that XML is well-formed and get a line-broken copy back when it parses cleanly.
How it works
The text is parsed with the browser's DOMParser in application/xml mode. If the parser produces a <parsererror>, its message is shown, trimmed to the first couple of hundred characters. If parsing succeeds, the document is re-serialised and a line break is inserted between adjacent tags so the structure is readable.
What “well-formed” means
Exactly one root element; every start tag has a matching end tag or is self-closing; tags are properly nested; attribute values are quoted; and the five reserved characters are escaped as < > & ' " where required. Well-formed is not the same as valid — validity is conformance to a DTD or XSD, which this tool does not check.
Worked example
Input <note><to>Ada</to><body>Hello</body></note> reports “XML is well formed.” and returns:
<note>
<to>Ada</to>
<body>Hello</body>
</note>Change it to <note><to>Ada<body>Hello</body></note> and it reports a mismatched-tag error because <to> is never closed.
Errors it catches
| Problem | Example |
|---|---|
| Mismatched or unclosed tag | <a><b>x</a> |
| More than one root element | <a/><b/> |
| Unquoted attribute value | <a id=1/> |
| Unescaped reserved character | <a>Tom & Jerry</a> |
| Undeclared namespace prefix | <ns:a/> with no xmlns:ns |
When to use it
Confirming that a config file, an RSS or Atom feed, a SOAP body, or an SVG parses before you feed it to something stricter, and locating an unclosed or misnested tag in a document a build tool rejected without a helpful line number.
Does it validate against a schema (XSD/DTD)?
No, only well-formedness.
Why does a stray & cause an error?
A bare & must be written & unless it starts a valid entity.
Does it check that namespaces are declared?
An undeclared namespace prefix on an element does raise a parser error.
Is my document uploaded?
No, DOMParser runs in the browser.