<style>
.container {
  margin: 200px;
  height: 300px;
  width: 500px;
}
.content {
  margin-left: -50px;
  margin-right: -50px;
  overflow: hidden;
  height: 50px;
  background-color: green;
}

.float {
  float:left;

  width: 100px;
  margin-left: -100px;

  height: 100px;
}
</style>
<!-- float's margin box is shrunk to zero so float avoider could simply "overlap" it -->
<div class=container><div class=float></div><div class=content></div></div>