I followed the advice of Dion and reduced the number of <h4> and other <h> headers to 15 by replacing them by <p> tags like <p class="4">...</p> with p.4 {....}.
But then I made the strange experience, that the browser didn't read any <p> style with number. I had to make it like
<h2> ---> p.2 ---> p.b
<h3> ---> p.3 ---> p.c
<h4> ---> p.4 ---> p.d
and so on.
The only tag with number that's left now is <h4>.
Now It worked. Apparently there was a conflict somewhere, but I didn't find it. And when the FlightCrew said: 'No problem found', I left it like that and the result looks fine. But I would like to know what was wrong.
Any suggestions?
George