The current LibreOffice behavior produces XHTML exports with an HTML
heading hierarchy not strictly and semantically corresponding to the
heading hierarchy of the ODF document.

Concretely LibreOffice produces XHTML exports with many <h1> headings.
While for an (X)HTML document to be semantically correct, there must be
one and only one <h1> per document. And the heading hierarchy should
then follow, <h2>, <h3>, etc. down maximum to <h6>.

The current LibreOffice behavior produces the following XHTML export of
the given ODT file:

    <p class="Title">Titre principal</p>

    <h1 class="Heading_20_1"><a id="a__Titre_1"><span/></a>Titre 1</h1>
    <p class="P1">para</p>

    <h2 class="Heading_20_2"><a id="a__Titre_2"><span/></a>Titre 2</h2>
    <p class="P1">para</p>

    <h3 class="Heading_20_3"><a id="a__Titre_3"><span/></a>Titre 3</h3>
    <p class="P1">para</p>

while we want to have the following:

    <h1>Titre principal</h1>

    <h2 class="whatever"><a id="a__Titre_1"><span/></a>Titre 1</h1>
    <p class="P1">para</p>

    <h3 class="whatever_else"><a id="a__Titre_2"><span/></a>Titre 2</h3>
    <p class="P1">para</p>

    <h4 class="whatever_again"><a id="a__Titre_3"><span/></a>Titre 3</h4>
    <p class="P1">para</p>


Technical explanation of the current LibreOffice bug:

Title has child Heading1. Heading1 has child Heading2. Heading2 has
child Heading3. etc. And Title is the root of the ODF heading hierarchy.
Es gibt:
Title→Heading1→Heading2→Heading3→etc.

As h1 is the root of the (X)HTML hierarchy, in XHTML the heading
hierarchy is:
h1→h2→h3→h4→etc.

The current LibreOffice behavior is the following:
Title→Heading1→Heading2→Heading3→etc.
=>
p→h1→h2→h3→etc.

while it should logically and semantically be:
h1→h2→h3→h4→etc.

This is a heading root mismatch between ODF and (X)HTML export

Changing this behaviour could be implemented as a filterOption that would postprocess the current output