[html] How to make div go behind another div?

One possible could be like this,

HTML

<div class="box-left-mini">
    <div class="front">this div is infront</div>
    <div class="behind">
        this div is behind
    </div>
</div>

CSS

.box-left-mini{
float:left;
background-image:url(website-content/hotcampaign.png);
width:292px;
height:141px;
}
.front{
    background-color:lightgreen;
}
.behind{
    background-color:grey;
    position:absolute;
    width:100%;
    height:100%;
    top:0;
    z-index:-1;
}

http://jsfiddle.net/MgtWS/

But it really depends on the layout of your div elements i.e. if they are floating, or absolute positioned etc.