.banner-topo {
  background: url('../img/banner-topo-erro-404.jpg') no-repeat center top;
  width: 100%;
  height: 400px;
  z-index: -1;
  -webkit-background-size:cover;
  -moz-background-size:cover;
  -o-background-size:cover;
  background-size:cover;
}

.limpa{clear:both}