Definition
HTML describes a document's structure and meaning as a tree of nested elements, which the browser parses into the DOM. Modern HTML is a living standard maintained by the WHATWG rather than a versioned spec, so features land continuously. Its element set is semantic by design: `<nav>`, `<article>`, `<main>` and `<header>` communicate intent to browsers, screen readers, and search crawlers in a way that a wall of `<div>` cannot. Attributes carry behavior and metadata—`alt` for accessible images, `loading="lazy"` for deferred fetching, ARIA attributes where native semantics fall short. Well-formed, semantic HTML is the cheapest accessibility and SEO win available, since it requires writing the correct tag rather than adding anything.