body {
  background-color: red;
}

#all {
  background-color: white;
  width: 600px;
  margin-left: auto;
  margin-right: auto;
}

header {
  text-align: center;
}

nav {
  background-color: green;
  float: left;
  width: 150px;
  color: white;
  height: 400px;
}

#content {
  background-color: yellow;
  float: left;
  width: 440px;
  height: 400px;
  padding-left: 10px;
}

footer {
  color: white;
  background-color: gray;
  clear: both;
  text-align: center;
}

@media (max-width: 600px) {
  /*special instructions for narrower screens */
      
   #all {
    display: block;
    width: 90%;
    font-size: 125%;
   }
   
   nav {
    display: block;
    width: 100%;
    height: auto;
   }
   
   #content{
    display: block;
    width: 95%;
    height: auto;
    padding-left: 5%;
   }
   
   footer {
    display: block;
    width: 100%;    
   }
   
}