How to format and validate XML

XML (eXtensible Markup Language) encodes structured data as nested elements with opening and closing tags. It's still everywhere: SOAP web services, RSS and Atom feeds, sitemaps, Office documents, Android layouts, Maven builds, SVG images and many enterprise and government data exchanges. This guide covers what makes XML well-formed, the errors you'll hit most often, and how to format and validate it.

Anatomy of an XML document

xml
<?xml version="1.0" encoding="UTF-8"?>
<order id="o1" status="shipped">
  <customer>Asha</customer>
  <items>
    <item sku="A-100" qty="2">Notebook</item>
    <item sku="B-200" qty="1">Pen &amp; ink set</item>
  </items>
  <note><![CDATA[Leave at the <back> door]]></note>
</order>

• The declaration (<?xml ... ?>) states the version and encoding. It's optional but recommended.
• Elements such as <customer> hold text or other elements.
• Attributes such as sku="A-100" hold simple values on an element.
• Entities such as & represent characters that have special meaning.
• CDATA sections contain text that isn't parsed, so <back> inside one is plain text.

Well-formed vs valid

• Well-formed XML follows the basic syntax rules. Every parser requires this.
• Valid XML is well-formed and matches a schema (XSD or DTD) that defines which elements and attributes are allowed. Validity only matters when a schema exists.

The rules for well-formed XML

1. Exactly one root element wraps the whole document.
2. Every opening tag has a matching closing tag, or is self-closing: <br/>.
3. Tags nest properly. <a><b></b></a> is fine; <a><b></a></b> isn't.
4. Names are case-sensitive. <Item> and </item> don't match.
5. Attribute values are quoted, with single or double quotes.
6. Special characters are escaped in text and attribute values.

Characters that need escaping

Character	Entity	Required?
<	&lt;	Always in text and attributes
&	&amp;	Always
>	&gt;	Recommended
"	&quot;	Inside double-quoted attributes
'	&apos;	Inside single-quoted attributes

An unescaped & is the single most common reason XML fails to parse, often in URLs (?a=1&b=2) or company names.

Common errors and what they mean

• "Opening and ending tag mismatch": a closing tag doesn't match, often due to a typo or wrong case.
• "Extra content at the end of the document": there's more than one root element. Wrap them in a single parent.
• "EntityRef: expecting ';'": an unescaped &.
• "XML declaration allowed only at the start": whitespace or a byte order mark appears before <?xml, often after concatenating files.
• Encoding errors: the declaration says UTF-8, but the file was saved in another encoding.

Format and validate XML in the browser

1. Paste your XML into the XML Formatter.
2. Click Format to indent it, or Minify to remove whitespace for transport.
3. If the XML isn't well-formed, the tool shows the parser's error message instead of formatting it.
4. To check validity, provide an XSD schema; see How to use the XML Formatter.

Formatting only changes whitespace between elements. Be careful with mixed content, where text and elements are interleaved (<p>Hello <b>world</b></p>). Adding line breaks there can change how the text is displayed.

Namespaces in one paragraph

Namespaces prevent name clashes when documents combine vocabularies: <xs:element xmlns:xs="http://www.w3.org/2001/XMLSchema">. The prefix (xs) is just a shorthand. What identifies the namespace is the URI. Two documents with different prefixes bound to the same URI are equivalent.

When to use XML vs JSON

New web APIs mostly use JSON, but XML remains the better fit for documents with mixed content, formats that need schemas, comments or namespaces, and systems that already standardize on it. See XML vs JSON: when to use which.