<a>The_button</a>
<div style="position:absolute">
    <div style="position:relative" class="inner-box">
       Content
       Content
       Content
       Content
       Content
    </div>
</div>

<div class=outer-box>
    The Button
    <div class=inner-box>
    </div>
</div>​

.outer-box {
    position: relative;
}

.inner-box {
    position: absolute;
    /* align bottom-right with offsetParent's top-left */
    bottom: 100%;
    right: 100%;
    width: 100px; /* fixed width, else contents will shrink */
}