.btn { border: none; background-color: inherit; padding: 5px 10px; font-size: 18px; cursor: pointer; display: inline-block; color: darkblue;}
.btn:hover {color: red;}
div{padding:2px;}
* { box-sizing: border-box;}  
body{text-align: center;font-family:Helvetica,Arial,sans-serif; font-size: 14px; background-color: beige; background-color:Linen; } #presentation{background-image: url("fond_develop.png"); width:690px;margin:auto;} 
#castelli{width:33%; float:left; height:335px;padding-left:10px;} #cours{width:33%; float:left;height:335px;} #notCours{width:100%; } #bulle{width:100%; height:100px; background-image: url("l1.png");}
div.total{width:100%;} div.tiers{float:left; width:33%; padding-top: 20px;}
#bulle{background-repeat: no-repeat;}
#lichtenstein{width:33%; float:left; height:335px;} #cadre{width:100%;}
#mediation{width:66%; float:left;} #videos{width:33%; float:right;}
a{font-size:18px;} div.padding{padding-top: 20px; width:100%;}
b.couleur1{color:brown;font-size:20px;} h1.couleur2{color:blue;}
u.bleu{font-size:18px;}
@media only screen and (max-width:600px){#presentation{width:100%;} #castelli{width:50%;} #cours{width:50%;} #lichtenstein{width:100%;} #fenetre{width:100%;} div.tiers{width:50%;} }