Wie man einen DIV über einem anderen DIV schweben lässt

<span><!</span><span>DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"</span><span>></span><span><</span><span>meta </span><span>http-equiv</span><span>="Content-Type"</span><span> content</span><span>="text/html; charset=gb2312"</span><span>/></span><span><</span><span>style </span><span>type</span><span>="text/css"</span><span>></span><span><!-- 
.d1 </span><span>{</span><span>        position</span><span>:</span><span>absolute</span><span>;</span><span>        top</span><span>:</span><span>20px</span><span>;</span><span>        left</span><span>:</span><span>20px</span><span>;</span><span>        width</span><span>:</span><span>200px</span><span>;</span><span>        height</span><span>:</span><span>200px</span><span>;</span><span>        background</span><span>:</span><span>red
        </span><span>}</span><span>.d2 </span><span>{</span><span>        width</span><span>:</span><span>500px</span><span>;</span><span>        height</span><span>:</span><span>100px</span><span>;</span><span>        background</span><span>:</span><span>blue</span><span>;</span><span>}</span><span>
-->
</span><span></</span><span>style</span><span>></span><span><</span><span>div </span><span>class</span><span>="d1"</span><span>></</span><span>div</span><span>></span><span><</span><span>div </span><span>class</span><span>="d2"</span><span>></</span><span>div</span><span>></span>