Creating a CSS3 box-shadow on all sides but one

Cut it off with overflow.

div div {box-shadow:0 0 5px #000; height:20px}
div {overflow:hidden;height:25px; padding:5px 5px 0 5px}

<div><div>tab</div></div>

In your sample create a div inside #content with this style

#content_over_shadow {
    padding: 1em;
    position: relative; /* look at this */
    background:#fff;    /* a solid background (non transparent) */
}

and change #content style (remove paddings) and add shadow

#content {
    font-size: 1.8em;
    box-shadow: 0 0 8px 2px #888; /* line shadow */
}

add shadows to tabs:

#nav li a {
    margin-left: 20px;
    padding: .7em .5em .5em .5em;
    font-size: 1.3em;
    color: #FFF;
    display: inline-block;
    text-transform: uppercase;
    position: relative;
    box-shadow: 0 0 8px 2px #888; /* the shadow */
}

You can use multiple CSS shadows without any other divs to get desired effect, with the caveat of of no shadows around the corners.

div.shadow {
    -webkit-box-shadow: 0 -3px 3px -3px black, 3px 0px 3px -3px black, -3px 0px 3px -3px black;
    -moz-box-shadow:    0 -3px 3px -3px black, 3px 0px 3px -3px black, -3px 0px 3px -3px black;
    box-shadow:         0 -3px 3px -3px black, 3px 0px 3px -3px black, -3px 0px 3px -3px black;
    height: 25px
}

 <div style="height: 25px"><div class="shadow">tab</div></div>

Overall though its very unintrusive.