What is XML?

XML uses nested tags to describe data: <book><title>Dune</title><author>Frank Herbert</author></book>. It was designed in the late 1990s as a simpler alternative to SGML and became the default interchange format for the early 2000s web. SOAP, RSS, Atom, sitemaps, SVG, and Office documents are all XML under the hood.

JSON ate XML's lunch in the 2010s for HTTP APIs because it is shorter, less ceremonious, and maps directly to JavaScript objects. XML is still where you find it locked into enterprise integrations (banking, healthcare, government), document formats (DOCX, EPUB, SVG), and SEO standards (sitemaps, RSS). Schemas (XSD) and transforms (XSLT) give XML capabilities JSON has only recently been catching up to with JSON Schema and JSONata.

For crawlers, XML matters because sitemaps are XML. The structure is well-defined (urlset, url, loc, lastmod, changefreq, priority) so a sitemap parser is one or two helper functions. RSS and Atom feeds are also XML and remain a clean way to discover new content on a site without crawling every page.