Semantic HTML
HTML that uses tags describing what each part of a page is, a heading, a list, a navigation block, an article, so a machine can tell structure from styling instead of seeing one flat wall of text.
You probably already read that you should "use semantic HTML." What that advice rarely says is that a machine reads your tags, not just your words.
What semantic HTML is
Semantic HTML is markup that names what each part of a page is. A heading is tagged as a heading. A list is tagged as a list. The main content sits in a main block, the menu in a nav block, the article in an article block.
The opposite is a page built almost entirely from generic containers. To a person it looks fine, because styling does the work. To a machine it is one undifferentiated slab with no signal about what is content and what is furniture.
Why AI engines lean on it
An engine fetching your page has to decide which text is the answer and which is the chrome around it. Semantic tags hand it that map for free.
A heading tag says this line introduces a section. A list tag says these items belong together. Without those tags, the engine is inferring structure from font sizes and spacing, which it cannot see the way you do.
The old way and the new way
The old way treated tags as a styling concern: pick whatever container, then make it look right with CSS. The page rendered correctly, so the job felt done.
The new way treats tags as meaning. The same page can look identical to a person and read very differently to a machine, depending entirely on which tags carry the content.
The damaging admission
Semantic HTML alone does not make a page worth quoting. You can tag everything correctly and still have a thin page that says nothing an engine wants.
And much of this lives in a theme or page builder you did not hand-code. Tidy tags on a page with no real answer just help a machine read confirmation that there is nothing here. Structure is a floor, not a finish.
How to check yours
Take one page and ask whether a machine could tell its parts apart: the main answer, the headings that frame it, the lists that group it, the navigation it should ignore. If everything is a generic box, the structure is invisible.
Run a free scan on any URL to see how an engine reads its structure today, then read the guide to optimizing headings for AI for the next layer.