<ul>
<li class="topNav">About Us
<ul class="subNav"><li> Subsection A</li><li>Subsection B</li></ul>
</li>
</ul>

.base:hover{position:relative;}

<ul id="a">
  <li id="b">b</li>
  <li id="c">c</li>
</ul>
<div id="d"></div>

background: url(blank.gif);

<div class="base" style="z-index:2">
<div class="inside">
    This has some more text in it. This also has a background. This should obscure the second block of text since it has a higher z-index.
</div>
This has some text in it. This has some text in it. This has some text in it. This has some text in it. This has some text in it.
</div>

<div class="base" style="z-index:1">
This is the second div. You should not be seeing this in front of the grey box. This has some text in it. This has some text in it. This has some text in it. This has some text in it. This has some text in it. This second box should be obscured by the grey box.
</div>