XML

XML (eXtensible Markup Language) is a text-based format for encoding structured documents with nested tagged elements. It was standardised by the W3C in 1998 and remains one of the most widely deployed data formats in the world — though its everyday visibility has declined as JSON took over the API layer.

Syntax

<?xml version="1.0" encoding="UTF-8"?>
<book id="978-0-13-468599-1">
  <title>The C Programming Language</title>
  <authors>
    <author>Brian Kernighan</author>
    <author>Dennis Ritchie</author>
  </authors>
  <published year="1978" />
</book>

Every piece of data is either:

• An element — <tag>content</tag> or the self-closing form <tag />.
• An attribute — id="..." on an opening tag.
• A text node — the text content of an element.

Elements can nest arbitrarily deep. A single root element must contain all others.

Where XML still dominates

• Office document formats. DOCX, XLSX, PPTX, ODF — all XML inside a ZIP archive.
• Enterprise integration. SOAP, HL7 (healthcare), FIX (finance), EDI/X12.
• Publishing. DocBook, TEI, JATS (journal articles).
• Configuration in the Java / .NET world. Maven pom.xml, Spring, .csproj.
• SVG — the entire vector-graphics format is XML.
• RSS and Atom syndication feeds.

What makes XML different from JSON

• Mixed content (text interleaved with child elements) is natural: <p>Hello <strong>world</strong>.</p>.
• Attributes are a separate concept from child elements, enabling meaningful modelling choices.
• XML namespaces allow mixing vocabularies.
• XSD provides stronger validation than JSON Schema for document-like data.
• A full transformation language (XSLT) exists; JSON has nothing equivalent.

What JSON has that XML doesn’t

• A native array type (XML requires repeated sibling elements by convention).
• Roughly half the byte count for the same data.
• Browser-native JSON.parse / JSON.stringify.
• A smaller surface area — six data types, simple rules.

For the comparison in depth, see JSON vs XML.