What it is
Every part of a page can be marked up two ways: generically, with anonymous <div> and <span> tags, or semantically, with tags that carry meaning. <header>, <nav>, <main>, <article>, <button>, and ordered heading levels say what a thing is. A <div> says nothing. Semantic HTML is choosing the tag that names the role, so the structure of the page is legible from the code alone.
Why it matters
Semantic HTML serves all three audiences at once: people, search engines, and AI systems. Screen readers use it to navigate, so it is an accessibility baseline. Search engines use it to understand hierarchy and importance. And AI systems and agents lean on it heavily, because a page built from meaningful tags can be parsed and acted on without guesswork. A wall of <div> tags forces every reader, human or machine, to infer what you could have simply stated.
What to do
Use heading levels in order, with a single <h1> for the page title. Wrap real navigation in <nav>, main content in <main>, and self-contained pieces in <article>. Use <button> for actions and <a> for links, never a styled <div>. Keep each tag describing the role, not the styling. Run WAIO Engine to flag pages leaning on generic tags where semantic ones belong.