Extensible Markup Language (XML) 1.0
XML remains embedded in enterprise integrations, document formats, and legacy APIs. Namespaces, entity expansion (XXE), and DTD processing are security-critical in parsers.
What It Defines
Text markup format: prolog (`<?xml version="1.0"?>`), elements, attributes, namespaces, entities, and DTD/schema validation. Basis for SVG, RSS, Atom, SOAP, SAML, OpenDocument inner XML, and many config files. MIME types: `application/xml`, `text/xml`.
Canonical (Normative)
Convenient (Practical)
Publishes web platform specs including CSS, accessibility, security policies, Service Workers, Web App Manifest, and many browser APIs. Also maintains some versioned HTML/DOM specs.
Related Specs
SVG powers icons, logos, charts, and interactive graphics. Because it's XML (not opaque binary), XSS and SSRF via embedded references are real risks in upload pipelines.
JSON is the lingua franca of web APIs. RFC 8259 is short and worth reading once — it clarifies edge cases around numbers, encoding, and trailing commas.
YAML configs power most DevOps tooling. Anchor/alias and implicit typing cause subtle bugs — know why `countries: [NO, US]` parses as `[false, null]` in YAML 1.1.
ODF is the default format for LibreOffice/OpenOffice and some government mandates. ZIP envelope + XML payload pattern mirrors OOXML design.
OOXML is the dominant enterprise document format. Every `.docx` is a ZIP you can unzip — knowing the part layout enables templating, mail-merge, and malware scanning.