OK many thanks.
I made some tests this morning... Works well for the above example
But doesn't work for this one :
<p>para 1</p>
<h2>title 2.1</h2>
<p>para 2.1</p>
<h2>title 2.2</h2>
<p>para 2.2</p>
-->
<para>para 1</para>
<section>
<title>title 2.1</title>
<para>para 2.1</para>
<section>
<title>title 2.2</title>
<para>para 2.2</para>
</section>
</section>
My example is not very HTML5 compliant, because of the lack of h1, but it is a fragment...