Markup Language

Markup language is a system for annotating a document so that its structure, formatting, and metadata are encoded with tags or symbols that are distinguishable from the running content. Rather than describing computation, a markup language describes what the content is — this is a heading, this is a link, this is a data field — so that both people and software can process it consistently. The family runs from SGML through HTML, XML, and lightweight descendants like Markdown.

• Structure over presentation - Tags label meaning and hierarchy, letting the same document be rendered, transformed, or queried many ways.
• Human- and machine-readable - The annotations are legible in the raw text yet unambiguous enough for parsers to consume.
• Standardized grammars - Formal specifications (SGML, XML, HTML) define valid syntax so tooling can validate and interoperate.
• A foundation, not a single format - “Markup language” is a category; concrete standards like XML and HTML are its members.

In API work markup languages are foundational plumbing: XML carried the first wave of web services and still backs SOAP, RSS/Atom feeds, and many enterprise payloads, while HTML remains how developer portals and documentation reach humans. Even as JSON and YAML dominate modern API definitions, the markup-language lineage shapes how we think about structured, self-describing data — and understanding that lineage helps teams govern the mix of formats an API estate inevitably accumulates.