@media only screen and (orientation:portrait){

  article:first-of-type:before {   
  background: #990099; 
  color:white;
  position:fixed;
  top:0;
  left:0;  
  width: 100%; 
  height:100%;  
  padding: 100px 10px 10px 10px;        
  font-size: 3.2rem;
  text-align:center;    
  content: "Merci de tourner votre appareil en mode paysage"; } 
}