XML Syntax Rules

XML enforces a small set of strict syntax rules that every document must follow exactly, which is what allows any XML parser to reliably read any well-formed XML document.

The Core Rules

These rules are non-negotiable — a document that breaks even one of them is not valid XML and will be rejected by any conforming parser.

• Every document must have exactly one root element.
• Every opening tag must have a matching closing tag (or be self-closing).
• Tags must be properly nested — never overlapping.
• XML is case-sensitive: <Name> and <name> are different tags.
• Attribute values must always be quoted.

Correct Nesting

Elements must close in the reverse order they were opened — this is one of the most common mistakes when hand-writing XML.

<!-- Correct -->
<book><title>Learning XML</title></book>

<!-- Incorrect - overlapping tags -->
<book><title>Learning XML</book></title>

Common Mistakes

• Overlapping tags instead of properly nesting them, e.g. closing a parent element before its child.
• Forgetting that XML is case-sensitive, so <Book> and <book> are treated as entirely different elements.
• Leaving an attribute value unquoted, e.g. writing id=42 instead of id="42".
• Including more than one root element at the top level of a document.