SOURCE

<section>
  <article></article>  
  <div>
    <span></span>
  </div>
</section>

section{
  height: 500px;
}
div{
  position: relative;
  width:100%;
  height:100%;
}
article{
  position:fixed;
  background:#000;
  top:0;
  left:0;
  width: 100%;
  height:100%;
  z-index:1;
}
span{
  background:#fff;
  display:block;
  width:50px;
  height: 50px;
  position: absolute;
  left:0;
}