XML Validator

Validate XML syntax

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XML Validator — Validate XML Syntax & Well-Formedness

Understanding the Basics

XML (eXtensible Markup Language) is a markup language defined by the W3C that uses nested tags to represent hierarchical data. Unlike HTML (which browsers render leniently), XML parsers are strict: a document is either "well-formed" (following all XML syntax rules) or invalid, with no error recovery. Well-formedness rules include: every opening tag must have a matching closing tag, tags must be properly nested (never crossing), attribute values must be quoted, the document must have exactly one root element, and reserved characters like < and & must be escaped as &lt; and &amp;. Beyond well-formedness, an XML document can also be "valid" — conforming to a Document Type Definition (DTD) or XML Schema (XSD) that constrains what elements and attributes are allowed. XML remains the dominant format in enterprise systems: SOAP web services, JEE application descriptors, Maven POM files, Microsoft Office OpenXML, SVG graphics, RSS/Atom feeds, and Android layout files all use XML. Validating XML before processing prevents entire classes of downstream parse failures and integration errors. Our validator checks well-formedness using the @xmldom/xmldom library and reports precise line/column error locations.

How to Use the Tool

  1. Paste your XML document into the editor.
  2. The validator checks well-formedness automatically, highlighting any structural errors.
  3. Error messages include the line number and column position of the offending character or tag.
  4. Fix the reported issue and the validation status updates in real time.
  5. For DTD or XSD validation, use the "Schema Validation" mode and provide your schema document.

Key Features & Specifications

  • Real-time well-formedness checking with line and column error positions
  • Detects unclosed tags, mismatched nesting, unquoted attributes, and illegal characters
  • Checks for exactly one root element (multiple root elements are invalid XML)
  • Validates XML declarations (<?xml version="1.0" encoding="UTF-8"?>)
  • Handles UTF-8, UTF-16, and ISO-8859-1 character encodings

Frequently Asked Questions (FAQ)

Q:What is the difference between well-formed and valid XML?
Well-formed XML follows the basic XML syntax rules (proper nesting, closed tags, quoted attributes). Valid XML additionally conforms to a schema (DTD or XSD) that defines which elements and attributes are allowed in which context. All valid XML is well-formed, but not all well-formed XML is valid.
Q:Why does my XML work in the browser but fail validation?
Web browsers parse HTML (which is lenient and error-recovering), not strict XML. Even XHTML served as text/html is parsed with HTML5 error correction rules. An XML parser applies zero error recovery — any syntax mistake is a fatal error.
Q:How do I include a less-than sign (<) inside XML content?
The < character must be escaped as the entity reference &lt; inside XML text content and attribute values. Similarly, & must be &amp;, > is recommended as &gt;, " in attributes as &quot;, and ' as &apos;.