XML: Elements, Attributes & Well-Formedness

XML (eXtensible Markup Language) is a text-based markup language for structured data. Unlike HTML's fixed tag set, XML is extensible -- authors define their own tag names and document structure fit to their specific data.

Elements & Attributes

<book id="42" available="true">
  <title>Dune</title>
  <author>Frank Herbert</author>
</book>

<!-- book, title, author are ELEMENTS -->
<!-- id and available are ATTRIBUTES on the book element -->

Well-Formedness Rules

• Exactly one root element containing everything else in the document.

• Every opening tag must have a matching closing tag.

• Tags must be properly nested, never overlapping.

• Tag and attribute names are case-sensitive -- <Book> and <book> are different names.

Escaping Special Characters

<!-- WRONG: < starts a new tag in XML's grammar -->
<note>5 < 10</note>

<!-- CORRECT -->
<note>5 &lt; 10</note>

<!-- For larger blocks of literal content, use CDATA instead: -->
<script><![CDATA[if (a < b) { doSomething(); }]]></script>

Well-Formed vs. Valid

Well-formed means the document follows XML's basic syntax rules regardless of content. Valid means it additionally conforms to a specific schema (a DTD or XSD) defining exactly which elements and attributes are allowed and how they nest.