body { font-family: Verdana; }

div.mainmenu {
    font-weight: bold;
    margin-top: 150px;
}

div.menu {
    float: left;
    clear: left;
    margin-left: 10px; 
}

div.popup a:hover { background-color: #faa; }
div.menu:hover div:first-child { border-bottom: none; }
div.menu div.popup { display: none; }
div.menu:hover div.popup { 
    display: block; 
    background-color: #99f; 
}


div.menu div {
    width: 175px; 
    background-color: #66f;
    padding: 5px;
    border: solid 2px blue;
}


div.popup div, div.popup a { display:block; text-decoration: none; margin: 2px 2px 2px 5px; }
div.popup div + div, div.popup a + a { margin-top: 10px; } 

div.content {
    float: left;
    margin-left: 20px;
    border: 1px dashed gray;
    /* Without a smaller width, the content doesn't break - so it jumps beneath the left hand nav */
    width: 75%;
}