What is an XML file?
XML (Extensible Markup Language) is a markup language designed for storing and transporting structured data. The W3C published the first XML specification in 1998 as a simplified, human-readable alternative to SGML. Unlike HTML, XML has no predefined tags — developers define their own element names to match their data domain. This extensibility made XML the foundation for dozens of specialized formats still in use today.
XML is both human-readable and machine-parseable, making it suitable for configuration files, data exchange protocols, document formats, and web services. While JSON has replaced XML for many web API use cases, XML remains dominant in enterprise systems, document formats (DOCX, XLSX, SVG), and regulated industries.
How to open XML files
- Any web browser — Built-in XML viewer with collapsible tree
- VS Code (Windows, macOS, Linux) — Syntax highlighting, formatting, XML Schema validation
- Notepad++ (Windows) — XML tree view with plugin
- XMLSpy (Windows) — Professional XML editor with visual designers
- IntelliJ IDEA / WebStorm — IDE-level XML support
Technical specifications
| Property | Value |
|---|---|
| Standard | W3C Recommendation (1998, XML 1.0; 2004, XML 1.1) |
| Encoding | UTF-8, UTF-16 (required); others declared in prolog |
| Validation | DTD, XSD (XML Schema), RELAX NG |
| Transformation | XSLT stylesheets transform XML to HTML, PDF, other XML |
| Querying | XPath expressions, XQuery language |
| MIME type | application/xml or text/xml |
Common use cases
- Configuration files:
web.config(ASP.NET),pom.xml(Maven),AndroidManifest.xml - Data exchange: SOAP web services, EDI, financial messaging (FIX, FpML)
- Document formats: DOCX, XLSX, PPTX, and ODT are ZIP archives containing XML files
- Vector graphics: SVG (Scalable Vector Graphics) is XML
- Syndication: RSS and Atom feeds are XML
- Sitemaps:
sitemap.xmlfiles for search engine indexing
XML document structure
<?xml version="1.0" encoding="UTF-8"?>
<catalog>
<product id="101">
<name>Widget A</name>
<price currency="USD">29.99</price>
<inStock>true</inStock>
</product>
</catalog>The optional XML declaration (<?xml ... ?>) appears first. Elements must be properly nested and closed. Attributes appear inside opening tags. XML is case-sensitive (<Name> and <name> are different elements).
XML vs JSON
| Feature | XML | JSON |
|---|---|---|
| Comments | ✅ <!-- comment --> | ❌ |
| Attributes | ✅ Key-value on elements | ❌ |
| Namespaces | ✅ (xmlns:) | ❌ |
| Schema validation | ✅ XSD, DTD | ✅ JSON Schema |
| Verbosity | More verbose | More concise |
| Browser parsing | Built-in DOMParser | Built-in JSON.parse |
Validation with XML Schema (XSD)
XSD files define which elements, attributes, and data types are allowed in an XML document. Enterprise systems use XSD to validate incoming data before processing. An XML document is well-formed if it follows XML syntax rules, and valid if it conforms to its associated schema.
XSLT transformations
XSLT (XSL Transformations) stylesheets transform XML into other formats — HTML for display, different XML vocabularies for system integration, or plain text for reporting. XSLT is a declarative language that matches XML nodes with templates and outputs the transformed result. It remains widely used in publishing workflows and legacy enterprise integration.