Chapters: JavaScript HTML & CSS JSON XML YAML ← Full TOC

📄 XML Syntax — Elements, attributes, namespaces, and well-formed rules

XML (eXtensible Markup Language) is a strict markup language for structured data. Unlike HTML, XML tags are user-defined. Every XML document must be well-formed: properly nested, closed tags, quoted attributes, and a single root element.

📋 Reference

Item	Syntax / Value	Description
<?xml?>	<?xml version="1.0" encoding="UTF-8"?>	XML declaration — recommended first line
Element	<tagName>content</tagName>	Basic building block — must be properly closed
Empty element	<br/> or <br></br>	Self-closing element
Attribute	<element attr="value">	Attribute values MUST be quoted
Root element	Single root wraps all other elements	Every XML doc has exactly one root
Case-sensitive	<Name> ≠ <name>	XML tag names are case-sensitive
Nesting	<a><b></b></a> — correct	Tags must be properly nested — no overlapping
Entity refs	< > & " '	Escape < > & " ' in content/attributes
CDATA	<![CDATA[ <unescaped content> ]]>	Literal text block — no escaping needed
Comment	<!-- comment text -->	XML comment — cannot contain --
Namespace	xmlns:ns="http://example.com"	Avoid name collisions across vocabularies
Namespace prefix	<ns:element>	Prefix maps to namespace URI
Default namespace	xmlns="http://example.com"	Default namespace for unprefixed elements
Processing instruction	<?pi-target data?>	Instruction to the XML processor
DOCTYPE	<!DOCTYPE root SYSTEM "file.dtd">	Link to Document Type Definition
Well-formed	All tags closed, properly nested, one root	Minimum requirement for valid XML
Valid	Well-formed + conforms to DTD or XSD	Validated against schema
Encoding	UTF-8 (default) / UTF-16 / ISO-8859-1	Character encoding — use UTF-8
Whitespace	Preserved inside elements	Whitespace is significant in XML content
PI	<?xml-stylesheet type="text/xsl" href="style.xsl"?>	Link XSLT stylesheet

💡 Example

<?xml version="1.0" encoding="UTF-8"?>
<!-- MyWebUniversity Student Database -->
<university xmlns="https://www.mywebuniversity.com/schema"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="https://www.mywebuniversity.com/schema students.xsd"
            version="2.0"
            created="2026-06-25">

  <department name="Computer Science" code="CS">

    <student id="1001" status="active">
      <name>
        <first>Alice</first>
        <last>Smith</last>
      </name>
      <email>alice@mywebuniversity.com</email>
      <gpa>3.95</gpa>
      <enrolled>2024-09-01</enrolled>
      <graduation/>  <!-- empty element — date not yet set -->

      <courses>
        <course code="CS101" credits="3" grade="A">
          <title>Introduction to Python</title>
          <instructor>Dr. Brown</instructor>
        </course>
        <course code="CS201" credits="3" grade="A-">
          <title>JavaScript &amp; Node.js</title>  <!-- & escaped as &amp; -->
          <instructor>Dr. Green</instructor>
        </course>
        <course code="CS301" credits="4" grade="A+">
          <title>Data Science with R</title>
          <notes><![CDATA[Student excelled at <ggplot2> and dplyr::filter()]]></notes>
        </course>
      </courses>

      <tags>
        <tag>dean-list</tag>
        <tag>scholarship</tag>
        <tag>remote</tag>
      </tags>
    </student>

    <student id="1002" status="active">
      <name><first>Bob</first><last>Jones</last></name>
      <email>bob@mywebuniversity.com</email>
      <gpa>3.72</gpa>
      <enrolled>2024-09-01</enrolled>
      <graduation/>
      <courses/>
      <tags/>
    </student>

  </department>

</university>

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