A collection of CSS hacks

transparency.css

/* For IE7 & 8 */
.transparent-black {
    /* IE7 needs an element to be positioned in order to apply a transparent background, this is a work-around */
    zoom: 1;
    background-color: #000;
    filter: alpha(opacity=70);
    /* Fill the parent container */
    position: absolute;
    top: 0;
    left: 0;
    height: 100%;
    width:100%;
}

/* For all other browsers - requires modernizr */
.rgba .transparent-black {
    background: rgba(0, 0, 0, 0.7);
}

transparency.html

<section style="position: relative;">
    <div class="transparent-black"></div>
    <!-- Other stuff that won't end up transparent -->
</section>