[support] JQuery makes z-index useless?

[sander-martijn]

I'm having a problem with z-index on an object.  I don't use jQuery - I 
know javascript quite well and don't see the point in learning another 
way of doing it.  But no matter what z-index I put this div at, elements 
burn through it.  I did some searching and the only hint I got was that 
it may be something JQuery is doing, but couldn't find a fix.  If I have 
to put some JQuery in my page just to get the z-index working that's 
fine but I'd rather not do the whole thing that way.  If it's not JQuery 
does anyone know what might be causing this behavior?  I tested it on 
various pages in the site, most of which have no custom javascript or 
other strangeness on them but see the same behavior - z-index seems to 
be completely ignored (including the default way of html working where 
the last element is on top).  Anyone else familiar with this? 

very straightforward example below:
css:
#theOverlay{
  height:1000px;
  width:1000px;
  top:0px;
  left:0px;
  background-color:#000;
  z-index:100; /* i've tried every number here imaginable */
}


html (just before </body>):
<div id="theOverlay"></div>