Test this piece of code. You'll understand Im sure. I sed borders on sublevels to emphasize the point.
<HTML>
<HEAD>
<style type="text/css">
DIV#Section DIV {border:1px solid #000000; padding: 10px; margin: 10px;}
DIV#Section DIV DIV {border: 1px solid #FFCC00;}
</style>
</HEAD>
<BODY>
<DIV id="Section">
<DIV>Sublevel 1
<DIV>Sublevel 2
<DIV>Sublevel 3</DIV>
</DIV>
</DIV>
<DIV>Sublevel 1
<DIV>Sublevel 2</DIV>
</DIV>
</DIV>
</BODY>
</HTML>
Regards
Bosko