CSS Stacking Contexts
HTML
<section id="s1">
<div id="d1"></div>
</section>
<section id="s2">
<div id="d2"></div>
</section>CSS
section { width:120px; padding:20px; margin:20px; border:1px dashed #aaa; position:relative; }
div { width:120px; height:120px; }
#d1 { background-color:#f00; }
#d2 { background-color:#0f0; }