Trying our some media selectors for screen width

gistfile1.css

@media all and (max-width: 320px){
  article.intro {
    width: 100%;
  }
  section {
    width: 100%; 
  }
}

@media all and (max-width: 640px) and (min-width: 321px){
  article.intro{
    float:left;  
    width:50%; 
  }
  section{
    float:left;
    width:50%;
  }
}