XML - rules of XML

 1. Every XML document must have a root element

  • All other elements must be nested within a single root (parent) element.

<bookstore>
  <book>...</book>
</bookstore>

 2. Tags must be properly closed

  • XML is strict about closing tags. Every opening tag must have a corresponding closing tag, or be self-closing.

<author>John</author>
<line-break /> <!-- Self-closing tag -->

 3. Tags are case-sensitive

  • <Book> and <book> are considered different. You must consistently use the same case.

<Title>Correct</Title>
<title>Incorrect if you opened with Title</title>

 4. Elements must be properly nested

  • Tags cannot overlap. Inner elements must be completely closed before the outer one.

<!-- Correct -->
<note><to>John</to><from>Jane</from></note>

<!-- Incorrect -->
<note><to>John<from>Jane</to></from></note>

 5. Attribute values must be quoted

  • All attribute values must be enclosed in single or double quotes.

<book genre="fiction" year="2023">
  ...
</book>

6. Use of a prolog (optional but recommended)

  • The XML declaration typically appears at the top of the file.

<?xml version="1.0" encoding="UTF-8"?>

 7. Special characters must be escaped

  • Use entity references for characters like <, >, &, ', ":

    • < → &lt;

    • > → &gt;

    • & → &amp;

    • ' → &apos;

    • " → &quot;

8. Whitespace is preserved

  • XML preserves spaces and line breaks inside elements unless specifically handled otherwise by the application parsing it.

 9. Comments follow a strict format

  • XML comments must be enclosed in <!-- --> and cannot contain -- inside.

<!-- This is a valid XML comment -->

 10. Custom tags are allowed

  • XML is extensible, meaning you can create your own tags, as long as they follow the rules above.