DOCBOOK-APPS: subtitle more important than title in html tags

Hi, I'm new in this list and I have not checked archives carefully yet but I
have some problems converting docbook into other formats. One of it is that
when I use title and subtitle together then they are rendered in HTML in
reversed order/meaning of tags. I mean title is H3 and subtitle is H2. Is this
a bug or expected behaviour? I use it that way:

<book>
  <bookinfor><title>MyApplication</title>
    ...
  </bookinfo>
  <part><title>Introduction</title>
    <chapter><title>History</title>
      <sect1><title>Earlier</title>
        <sect2><title>16 August 2000</title><subtitle>Major update</subtitle>
          ...
        </sect2>
        ...
      </sect1>
      ...
    </chapter>
    ...
  </part>
</book>

I use docbook XSL 1.57.0 with xsltproc.

Wlodzimierz Skiba