Just played a bit, and this also seems to work
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8"/>
</head>
<body>
<p>Here’s a first footnote<a class="sdfootnoteanc" name="sdfootnote1anc" href="#sdfootnote1sym">1</a>. Here’s another footnote.<a class="sdfootnoteanc" name="sdfootnote2anc" href="#sdfootnote2sym">2</a></p>
<div id="sdfootnote1">
<p class="sdfootnote"><a class="sdfootnotesym" name="sdfootnote1sym" href="#sdfootnote1anc">1</a>Footnote 1.</p>
</div>
<div id="sdfootnote2">
<p class="sdfootnote"><a class="sdfootnotesym" name="sdfootnote2sym" href="#sdfootnote2anc">2</a>Footnote 2.</p>
</div>
</body>
</html>
LibreOffice seems to require to use these classes and id-formatting.