XML Guide

XML remains useful when structure and metadata need to be explicit

XML is verbose compared with JSON, but its strict nesting rules and attribute model still make it valuable for document-heavy and enterprise integrations.

Core concepts

Most XML mistakes come from breaking a small set of structural rules.

Elements

The main building blocks of the document, defined with start and end tags.

<title>Hello</title>

Attributes

Small pieces of metadata attached directly to an element.

<book id=\"123\">

Nesting

Elements must close in the reverse order they open so the tree stays valid.

<root><child /></root>

Prolog

An optional header can declare the XML version and encoding.

<?xml version=\"1.0\"?>

Syntax rules

XML parsers are intentionally strict, so malformed markup usually fails fast.

Every document needs exactly one root element that contains everything else.
Tags are case-sensitive, so <Book> and <book> are different names.
Every opened element needs a matching closing tag unless it is self-closing.
Attribute values must be quoted.
Reserved characters like <, &, and > must be escaped in text nodes where needed.

Typical places XML still shows up

RSS and Atom feeds
Sitemaps and search-related metadata
SOAP integrations and enterprise messaging
Office document bundles such as modern spreadsheet and document formats
Configuration systems that rely on schemas or rich markup

XML vs HTML

These formats share angle-bracket syntax, but they solve different problems.

Feature	XML	HTML
Purpose	Transport and store structured data	Render documents and interfaces
Tag set	User-defined	Predefined browser vocabulary
Error tolerance	Strict	Browsers recover from many mistakes
Closing tags	Required or self-closing	Some elements are implicitly closed

Example document

library.xml

<?xml version="1.0" encoding="UTF-8"?>
<library>
  <book id="1">
    <title>The XML Guide</title>
    <author>Jane Smith</author>
    <price currency="USD">29.99</price>
  </book>
  <book id="2">
    <title>Web Standards</title>
    <author>John Doe</author>
    <price currency="EUR">24.50</price>
  </book>
</library>