What is XML (eXtensible Markup Language)?
XML looks like HTML's stricter cousin. Data is wrapped in tags, an opening tag and a closing one, nested inside each other, but the tags are not fixed: whoever designs the format invents them. A product feed might use tags called product, name, price and image; a sitemap uses url, loc and lastmod. The rules are strict (every tag must close, the nesting must be correct, the document must be well-formed) so that any software can read it reliably. It became a W3C standard in 1998.
For a decade XML was the default way for systems to exchange data, and it is still everywhere, often unseen. XML sitemaps tell search engines about a site's pages. RSS, Atom and podcast feeds are XML. SVG images are XML. Word, Excel and PowerPoint files are zipped folders of XML. And in post-production, an edit often travels from one application to another as an XML timeline.
Why it matters
Why XML matters
Most people working on a website meet XML through the things it carries rather than the language itself, and those things matter. The XML sitemap is one of the main ways a site tells search engines what exists and what has changed. Feeds built on XML syndicate content, power podcasts and send product catalogs to shopping platforms. Many business systems, from finance to publishing, still exchange data in XML. And when any of these files is malformed, a single unclosed tag can stop the whole thing from being read, quietly, with no error anyone sees.
In practice
How XML works in practice
- 01
On most sites, XML files are generated automatically. A content management system or SEO plugin produces the sitemap and the feeds from the site's content, and e-commerce platforms generate product feeds. The team's job is configuration: which pages belong in the sitemap and which do not, what each feed includes, and making sure the files are listed where search engines and partners expect them, such as the sitemap's address in robots.txt.
- 02
Then the files are checked, because generators make mistakes. A sitemap is submitted in Google Search Console, where errors and excluded pages are reported. Feeds are run through validators before launch and after any change to the templates that produce them. When XML is exchanged with another system, the format is documented with a schema, which defines the allowed tags and structure so both sides can test against the same rules.
Watch out
What people get wrong about XML
Thinking it is obsolete because newer systems prefer JSON. JSON did take over most web APIs, but XML still runs sitemaps, feeds, documents and countless business integrations, and is not going anywhere. Second, editing generated XML by hand, so the next automatic update overwrites the fix. Third, ignoring encoding: an unescaped ampersand or a stray character in a copied product name is enough to make a whole feed invalid, which is why content going into XML is cleaned on the way in.
