HTML structures content using nested tags: `<h1>` for top-level headings, `<p>` for paragraphs, `<a>` for links, `<img>` for images. Browsers parse the tags and render them.
It's a description of structure and meaning, not appearance. Appearance is CSS's job. Behavior is JavaScript's. Together they're the three pillars of the web.
Modern HTML5 added semantic tags (`<article>`, `<nav>`, `<main>`) that help search engines and accessibility tools understand a page without guessing.