<style>
.a {z-index:0; width:350px; background:gray;}
.b {margin:25px; background:white;}
.c { z-index:1; position:absolute; left:20px; top:0px; width:200px; height:150px; background:red;}
</style>

<div class="a">
<iframe class="b"></iframe>
</div>
<div class="c"></div>