There is a huge conceptual difference between the styles concept as it is commonly accepted in the typesetting and design world and the header levels implemented in html. Styled concept is based on the full customisability, html is not. Just as in most other aspects, html inventors tried to mock up something good, but as usually screwed it up due to their limited understanding of the concept and/or simplification of the implementation. For example, as we are talking about headers, there are only 6 strictly sequential levels allowed.
Later, some heavy artillery expansion of the html (popularly known as CSS) added huge amount of doubling functionality together with redundant expressiveness, some new ambiguities and open holes in possibilities here and there. But the headers left as they were, plain dumb and ridiculously outdated. While they could add an CSS attribute for the structured level of the element, they didn't. So now, the html is proudly named of being structured if it has header elements. Great for headers. But what about the paragraph text? Illustrations? What level they belong too? W3C can't give you an answer. To the last header probably... Well, this is not how structuring is done. Even RTF is ages ahead, though it is actually an older format.
Sorry for my rant, but I think not everyone understand how utterly bad html actually is. All I said is just a tiny microscopic piece of this enormous hype of misconceptions commonly called html.
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