XML Basics Cheat Sheet

A foundational reference for XML document structure, elements, attributes, namespaces, and validation with XSD and DTD.

1 PageBeginnerFeb 2, 2026

Document Structure

Declaration, elements, and attributes.

xml

<?xml version="1.0" encoding="UTF-8"?><library>  <book id="b1" available="true">    <title>The Pragmatic Programmer</title>    <author>David Thomas</author>    <price currency="USD">39.99</price>  </book>  <!-- This is a comment --></library>

Namespaces

Avoiding element name collisions across vocabularies.

xml

<root xmlns:h="http://www.w3.org/TR/html4/"      xmlns:f="https://example.com/furniture">  <h:table>    <h:tr><h:td>Apples</h:td></h:tr>  </h:table>  <f:table>    <f:width>80</f:width>  </f:table></root>

XPath Basics

Selecting nodes within an XML document.

xml

/library/book              <!-- all book children of library -->//title                     <!-- all title elements, any depth -->/library/book[@id='b1']     <!-- book with id attribute b1 -->/library/book[1]             <!-- first book element -->//book/author/text()          <!-- text content of author -->//price[@currency='USD']       <!-- price elements with attribute -->

Key Concepts

Core terminology and validation approaches.

Well-formed- Every opening tag has a matching closing tag and tags are properly nested
Valid- Well-formed AND conforms to a schema (DTD or XSD)
Attribute vs. element- Attributes hold metadata on a tag; elements hold structured/nested content
CDATA section- `<![CDATA[ raw text ]]>` prevents special characters from being parsed as markup
Entity references- `<` `>` `&` `'` `"` escape reserved characters
XSD- XML Schema Definition; defines allowed structure, types, and constraints for validation