SOURCE

<div id="header"></div>
<div id="middle">
  <div id="sidebar">
    
  </div>
  <div id="main">
    
  </div>
</div>
<div id="footer"></div>

#header {
  height: 50px;
  background-color: blue;
}

#middle {
  height: 100%;
}

#sidebar {
  width: 100px;
  height: 300px;
  background-color: green;
  /* opacity: 0; */

  
#sidebar:hover {
  background-color: red;
}