Top Menu & Open Left-side Navigation

Here we have a menu bar at the top (please ignore its look and concentrate on its behaviour) and all the children of the selected menu item on the left side. All menu items on the left side are open (see Editor item).

The HTML-Code of this page looks like this:

  <div id="head">
    <div class="sitemenu">
      <ul><?php get_i18n_navigation(return_page_slug(), 0, 0); ?></ul>
    <div style="clear:both"></div>
  <div id="main">
    <div id="navigation">
      <ul><?php get_i18n_navigation(return_page_slug(), 1, 100, true); ?></ul>
    <div id="content">
      <h1 id="pagetitle"><?php get_page_title(); ?></h1>
      <?php get_page_content(); ?>
    <div id="footer">

The (functionally) relevant entries in the CSS are:

.sitemenu { position:relative; }
.sitemenu li { float:left; }
.sitemenu li.current > a { font-weight:800; } 
.sitemenu li.currentpath > a { font-weight:800; }

Note that this page is not shown in the menu, but as its parent is the Navigation page, the Navigation menu item is marked as current.