html, body{
width:100%;
height:100%;	/*must be added*/
margin:0px;
padding:0px;
}

#conteneur{
width:80%;
height:100%;	/*must be added*/
/*background-color: gray;*/		/*so not visible*/
margin:auto;
}

#haut{
height:20%;
background-color: yellow;
}
#milieu{
height:65%;		/*to make 100% by adding*/
overflow:auto;
background-color:green;
}
#bas{
height:15%;
background-color:blue;
}

#haut_in, #milieu_in, #bas_in {
/*padding:30px;	*/				/*doesn't modify width of outer div*/
}

#haut, #milieu, #bas, #haut_in, #milieu_in, #bas_in {
margin:0px;
}

#haut, #milieu, #bas {
padding:0px;
}

#haut_in *:first-child, #milieu_in *:first-child, #bas_in *:first-child {
margin-top:0px;
}

#haut_in *:last-child, #milieu_in *:last-child, #bas_in *:last-child{
margin-bottom:0px;
}