📖 XML Validation Guide

The Core Principles of XML

**XML (Extensible Markup Language)** is a markup language defined by the W3C. Unlike HTML, XML has no predefined tags. Users define their own tags and tag structures, making it extremely flexible for describing data representations across enterprise systems.

Well-Formed XML Constraints

XML parser engines are notoriously unforgiving. If a document violates the rules, parsing halts immediately. Ensure your XML is well-formed:

• Single Root Tag: There must be exactly one parent root element enclosing all other child nodes. Double roots are prohibited.
• Matching Tag Casing: XML tags are case-sensitive. <Item> must be closed with </Item>, not </item>.
• Proper Nesting: Tags must close in the reverse order they were opened. <a><b></b></a> is correct; <a><b></a></b> is invalid.
• Attribute Quotation: All attributes must be wrapped in double or single quotes. <item price=12> is invalid; it must be <item price="12">.

XML Entity Escaping Reference

To include characters that define XML syntax structure in your text contents, use these entities:

• & → &
• < → <
• > → >
• " → "
• ' → '