<div class="container">
  <div class="left">1</div>
  <div class="right">4</div>
  <div class="center">
    <div class="center-block">2</div>
    <div class="center-block" contenteditable="true">3<br><br><br><br><br></div>
  </div>
  <div class="float">5</div>
</div>

.left, .right{
  width: 100px;
  background: yellow;
}
.left{
  float: left;
  height: 100px;
}
.right{
  float: right;
  height: 60px;
}

.center{
  margin-right: 100px;
}
.center-block{
  background: green;
  float: right;
  width: calc(100% - 100px);
}
.float{
  float: left;
  max-width: 100px;
  background: blue;
}

getSelection().selectAllChildren(
  document.querySelector('[contenteditable]')
)