HTML

HyperText Markup Language — the document language of the web: elements that say what a piece of content is (a heading, a list, a button, a form field), which the browser turns into a DOM, an accessibility tree and a default rendering before any CSS or JavaScript arrives

Maintained by the WHATWG as a living standard, with the W3C publishing snapshots. The elements carry meaning, not appearance: a <button> is focusable, keyboard-operable and announced as a button for free, and a <div> styled to look like one is none of those things. That is why semantic HTML is most of accessibility and the whole of progressive enhancement's base layer.