.btn { border: none; background-color: inherit; padding: 10px 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;} #presentation{background-image: url("fond_deb.png");width:700px; background-color:Linen;margin:auto;} 
#tomorrow{width:33%; float:left; height:140px;} #cours{width:33%; float:left;height:140px;} #assemblage{width:33%; float:left; height:140px;} #archive{width:33%; float:left;} #hamilton{width:66%; float:left;} #lien_archive{width:24%; float:left;} #sofia{width:50%; float:left;} #duchanp{width:24%; float:left;} fenetre{width:100%;}
a{font-size:18px;} 
h1.couleur1{color:firebrick;} h1.couleur2{color:black;}
u.bleu {font-size:18px;}
@media only screen and (max-width:600px){#presentation{width:100%;} #fenetre{width:100%;} }