DOCBOOK-APPS: how to recognize depth of section for CSS

Whe I apply any of xhtml outputs from XSL 1.58 to the sections then they are
outputed into file as

  <div class="section">....</div>

I want then apply intendation to this rendering with

div.section {
  margin-left:20px;
}

This cause that every next level section level has larger margin. It works
nice. Now I want to customize it so margin would increase for sections at
<h1>, <h2> and <h3> level but for <h4> and deeper it should be the same as for
<h3>. I suppose I could do it using sect1, sect2, sect3... but there is a
problem. We move content from time to time and change level of section. It
could be hard to modify sect numbering each time we move content. Much usefull
it could be if XSL templates could investigate section level and output it as:

<div class="section_1">
  <h1>title1</h1>
  <div class="section_2">
    <h2>title2</h2>
    <div class="section_3">
      <h3>title3</h3>
      <div class="section_4">
        <h3>title4</h4>
      </div>
    </div>
  </div>
</div>

Any advice how to customize XSL for such output ?

ABX