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No matter whether they are involved or not, if you have a very long text
on one page as I have (which also has some advantages)
the use of <div>s helps me to use the right headings because it let me
understand whether a part of the page is a subsection or a subsubsection
and so on. But once I have used the different <div>s how could I style
them?
You don't have to! One possibility would be to style all sections the
same, all subsections same (but different from the sections), all
subsubsections same again (but again different from the sections and
subsections).
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There is only 1 section on each page.
On the page
https://www.scaiecat-spa-gigi.com/it/svezia.html
I have several subsections, for example a subsection for the menu on the
left side
which includes links to other pages of the same website, a subsection for
the summary,
another for the main content and so on.
The main content is divided in several subsubsections.
The page is very long and styling the different subsubsections might help
the reader to see
where one begins and where it ends.