How semantic HTML works
Every page is a stack of elements, and each element either carries meaning or does not. A div and a span carry none — they are containers you style. A header, nav, main, article, aside or footer element says what the block is for. A button element says a thing can be pressed. A label element says which text belongs to which form field. Semantic HTML simply means picking the element that describes the content, then styling it however you like.
The meaning is not decoration. Browsers expose it as an accessibility tree that screen readers navigate by: a user can jump straight to the main content, or list every landmark on the page, because the markup declared them. Keyboard behaviour comes free too — a real button takes focus and responds to Enter and the space bar, while a styled div with a click handler does neither unless someone rebuilds all of it by hand.
Why semantic HTML matters
For search, the gain is interpretation rather than a ranking bonus. A crawler that can tell your article body from the navigation, the sidebar and the footer has a much easier job deciding what the page is about, and that clarity carries through to the snippets and answers built from your text. Nothing about semantic markup boosts a ranking by itself; it removes the ambiguity that makes a page harder to understand.
The stronger argument is accessibility. A site built from meaningless containers is close to unusable with a screen reader, and in several of the markets small businesses here sell into, accessibility is treated as an obligation rather than a nicety. It also makes the page cheaper to maintain, because structure and appearance stop being tangled together.
Common mistakes with semantic HTML
Most damage now comes from page builders. Dragging blocks around produces deeply nested containers where a heading is a styled paragraph, a menu is a list of divs, and a form’s submit control is an anchor with JavaScript attached. The page looks correct and behaves badly for anyone not using a mouse.
Choosing heading levels by size is the other classic: an author wants smaller text, so a second-level heading becomes a fourth-level one and the document outline stops making sense. Beyond that, sites often declare several navigation regions with no labels to tell them apart, wrap everything on the page in a single main element including the header and footer, or use a table for layout — which forces a screen reader to announce rows and columns that mean nothing.
Getting it right
Read the page with the stylesheet switched off. If the order and the structure still make sense as a plain document, the markup is close to right; if it collapses into an unreadable pile, the meaning was living in the CSS. Then check that headings descend in order and describe sections rather than set type size — the same discipline covered under heading tags.
Try the page with the keyboard alone, tabbing through every control, and fix anything that cannot be reached or activated. These checks belong in the build, not in an audit afterwards, so raise them when you are commissioning web design and development and treat them as part of web accessibility rather than as an SEO extra.