XML Formatter & Validator
Normalize XML indentation and catch common mistakes like unclosed or mismatched tags. Runs entirely in your browser.
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How XML Formatter & Validator Works
What is XML?
XML (Extensible Markup Language) is a tag-based format for representing structured data — every piece of data is wrapped in a named element, written as an opening tag and a matching closing tag, optionally carrying attributes. It predates JSON and YAML as a data-interchange format and remains widely used for document formats (like SVG and DOCX internals), configuration files, SOAP APIs, RSS/ Atom feeds, and countless enterprise systems still built around it. This tool formats and lexically checks XML text — it doesn't execute, parse into live objects, or validate it against any schema.
XML vs JSON
JSON is generally more compact and maps naturally onto native data structures (objects, arrays, strings, numbers) in most programming languages. XML is more verbose but has native support for attributes (metadata on an element, distinct from its content), mixed content (text interleaved with child elements), comments, namespaces, and processing instructions — features JSON has no equivalent for at all. Neither format is strictly "better"; XML tends to win for document-like data (attributes and mixed content matter), while JSON tends to win for pure data interchange (arrays and objects matter, verbosity doesn't).
XML vs YAML
Both YAML and XML can represent deeply nested structured data, but their syntax philosophies are opposite: YAML relies on indentation and minimal punctuation for a very human-readable result; XML relies on explicit opening and closing tags, which is more verbose but self-delimiting — a tag's extent is unambiguous regardless of whitespace, which is exactly why XML doesn't suffer from the indentation-sensitivity mistakes YAML is prone to. See the YAML Formatter & Validator for the YAML side of that comparison.
Why Formatting Matters
Unlike YAML, XML's structure doesn't strictly depend on whitespace — but a densely packed, inconsistently indented XML document is still far harder to scan for a missing or misplaced closing tag than a cleanly indented one. Consistent formatting turns "does this document nest correctly" from a mental exercise into something visually obvious at a glance.
XML Well-Formedness
A "well-formed" XML document follows the basic syntax rules regardless of what the tags actually
mean: exactly one root element, every opening tag has a matching closing tag (or is self-closing),
tags nest properly without crossing (<a><b></a></b> is invalid —
elements must close in reverse order), and attribute values are quoted. This tool checks exactly these
lexical well-formedness rules — it has no concept of "valid" beyond that, which is a separate,
schema-dependent question (see XSD, DTDs) this tool deliberately doesn't attempt.
Common Mistakes
- Forgetting to close a tag. Every opening tag needs either a matching
</tag>or must be self-closing (<tag />) — a single missed closing tag can make everything after it structurally ambiguous. - Crossing tags instead of properly nesting them.
<a><b>text</a></b>is invalid —<b>was opened after<a>, so it must close before<a>does. - Multiple root elements. A well-formed XML document has exactly one top-level element wrapping everything else — two separate top-level elements side by side isn't valid XML, even though each one individually is well-formed.
- Unquoted or mismatched-quote attribute values.
id=1instead ofid="1", or mixing single and double quotes incorrectly, breaks parsing.
XML Security (XXE Overview)
XML has a well-known vulnerability class called XXE (XML External Entity) injection: some XML parsers, if configured to resolve external entities, will fetch a URL or read a local file referenced inside a document's DOCTYPE declaration — turning a parsed document into a way to read arbitrary files or make the parser issue network requests on an attacker's behalf. This tool has zero exposure to that class of risk, structurally: it's a lexical formatter, not a real XML parser, and never resolves entities, never loads DTDs, and never fetches anything, regardless of what a DOCTYPE declaration in your input claims. When working with a real XML parser in your own code, always disable external entity resolution and DTD loading unless you specifically need them and trust the source completely.
Best Practices
- Always include an XML declaration (
<?xml version="1.0" encoding="UTF-8"?>) at the very start of a standalone document, so consumers know the encoding without guessing. - Prefer attributes for metadata, elements for content — a common convention that
keeps documents predictable (e.g.
idas an attribute, the element's text as the actual value). - Disable external entity resolution in any real parser you use, even if you don't think you need it — it closes off an entire vulnerability class for free.
Related Tools
Working with structured data formats often overlaps with other developer utility tasks: reformat a JSON payload with the JSON Formatter, beautify a YAML config with the YAML Formatter & Validator, clean up a SQL query with the SQL Formatter & Validator, validate a pattern used to extract values from XML with the Regex Tester, generate a request snippet for a SOAP/XML API with the OpenAPI Snippet Generator or the cURL Command Builder.
Accuracy & Sources
Last reviewed: August 2026. Formula source: W3C Extensible Markup Language (XML) 1.0. All calculations run in your browser. No data is sent to any server.
Frequently Asked Questions
No — it's a lexical, tag-stack-based formatter, not a real XML parser. It never executes anything, never resolves external entities, never loads DTDs, never fetches external resources, and never validates against an XML Schema. None of that machinery exists in this tool at all.
Yes, by construction — XXE requires a parser that resolves external entities or loads DTDs, and this tool does neither. It's a plain text formatter, not an XML deserializer, so there's no entity-resolution machinery here to exploit regardless of what a DOCTYPE declaration in your input claims.
No — it only checks lexical well-formedness (matching tags, one root element, quoted attributes). It has no concept of schema validation and doesn't know what elements or attributes your specific application expects.
Warnings (unclosed tags, mismatched closing tags, multiple root elements, a duplicate XML declaration) describe issues found in your original XML, not the formatter's own doing. The tool still produces its best-effort formatted output alongside the warning, so you can see the document clearly enough to find and fix the actual problem.
Yes — comments and CDATA blocks are preserved exactly as written and re-indented to match their position in the document, without touching their internal content.
Well-formed means the basic syntax rules are followed — matching tags, one root element, quoted attributes — regardless of what the tags mean. Valid means the document additionally conforms to a specific schema (like an XSD) that defines which elements and attributes are allowed and in what order. This tool checks well-formedness only; schema validation is a separate, application-specific concern it doesn't attempt.