HTML, HyperText Markup Language, is the markup language that structures a web page: headings, paragraphs, links, images, tables, forms, each wrapped in tags that tell a browser what kind of content it's looking at. It's the skeleton every page is built on, with CSS handling appearance and JavaScript handling behavior layered on top of it.
Search engines read HTML directly to understand a page's content and structure, which is why the semantic accuracy of the markup matters beyond just how a page looks: a heading wrapped in an actual <h1> tag carries more topical weight than the same text styled to look like a heading using CSS alone, and a link needs a real <a href> tag to be followed and counted as a link at all. Clean, semantic HTML, structured content in the tags meant for that content rather than generic <div>s styled to resemble it, gives search engines and assistive technology alike a far clearer read on what a page actually contains.
