Merci!
voici le code CSS.
Ce qui me pose problème c'est au niveau des 3 rubriques, gauche,centre et droite.
Quand je ne met pas les positions, le pied de page est bien tout en bas. 
body
{
	margin: 0px 0px 0px 0px;
	padding: 0px 0px 0px 0px;
	font-family: verdana;
	font-size: 12px;
	background-image: url('background.jpg');
	background-attachment:fixed;
}
#entete
{
	margin: 0px 0px 0px 0px;
	padding: 0px 0px 0px 0px;
}
#contenu_gauche
{
	float: left;
	/*position: absolute;*/
	left: 15px;
	top: 270px;
	width: 250px;
	height: 300px;
	border: 1px solid grey;
	padding-left: 20px;
	border-radius: 20px;
	box-shadow: 7px 7px 7px black;
	background: -moz-linear-gradient(top,  rgba(255,255,255,1) 0%, rgba(255,255,255,0) 100%); /* FF3.6+ */
	background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(255,255,255,1)), color-stop(100%,rgba(255,255,255,0))); /* Chrome,Safari4+ */
	background: -webkit-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Chrome10+,Safari5.1+ */
	background: -o-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Opera 11.10+ */
	background: -ms-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* IE10+ */
	background: linear-gradient(to bottom,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* W3C */
	filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#ffffff', endColorstr='#00ffffff',GradientType=0 ); /* IE6-9 */
}
#contenu_droite
/* c'est un tableau*/
{
	float: right;
	/*position: absolute;*/
	top: 270px;
	right: 15px;
	width: 250px;
	border: 1px solid grey;
	border-radius: 20px;
	box-shadow: 7px 7px 7px black;
	height: 300px;
	background: -moz-linear-gradient(top,  rgba(255,255,255,1) 0%, rgba(255,255,255,0) 100%); /* FF3.6+ */
	background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(255,255,255,1)), color-stop(100%,rgba(255,255,255,0))); /* Chrome,Safari4+ */
	background: -webkit-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Chrome10+,Safari5.1+ */
	background: -o-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Opera 11.10+ */
	background: -ms-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* IE10+ */
	background: linear-gradient(to bottom,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* W3C */
	filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#ffffff', endColorstr='#00ffffff',GradientType=0 ); /* IE6-9 */
}
td
/* Reglages pour le tableau du contenu droite*/
{   margin: 0px;
	padding: 10px 10px 10px 10px;
	font-size: 12px;
}
#contenu_centre
/* Section nos principaux objectifs*/
{
	float:left;
	border: 1px solid grey;
	width: 550px;
	border-radius: 20px;
	box-shadow: 7px 7px 7px black;
	/*position: absolute;*/
	left: 360px;
	top: 270px;
	padding-left: 20px;
	margin-left:120px;
	margin-right:20px;
	margin-bottom:30px;
	background: -moz-linear-gradient(top,  rgba(255,255,255,1) 0%, rgba(255,255,255,0) 100%); /* FF3.6+ */
	background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(255,255,255,1)), color-stop(100%,rgba(255,255,255,0))); /* Chrome,Safari4+ */
	background: -webkit-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Chrome10+,Safari5.1+ */
	background: -o-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Opera 11.10+ */
	background: -ms-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* IE10+ */
	background: linear-gradient(to bottom,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* W3C */
	filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#ffffff', endColorstr='#00ffffff',GradientType=0 ); /* IE6-9 */
}
.contenu_rubrique_condition_vente, .contenu_rubrique_societe, .catalogue_produits
/* Section condition de vente, societe, decouvrez nos produits*/
{
	text-align: justify;
	border: 1px solid grey;
	border-radius: 20px;
	box-shadow: 7px 7px 7px black;
	width: 500px;
	/*position: absolute;*/
	left: 300px;
	top: 250px;
	padding: 0px 15px 0px 15px;
	margin-top: 20px;
	background: -moz-linear-gradient(top,  rgba(255,255,255,1) 0%, rgba(255,255,255,0) 100%); /* FF3.6+ */
	background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(255,255,255,1)), color-stop(100%,rgba(255,255,255,0))); /* Chrome,Safari4+ */
	background: -webkit-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Chrome10+,Safari5.1+ */
	background: -o-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Opera 11.10+ */
	background: -ms-linear-gradient(top,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* IE10+ */
	background: linear-gradient(to bottom,  rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* W3C */
	filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#ffffff', endColorstr='#00ffffff',GradientType=0 ); /* IE6-9 */
}
#piedepage
{
	clear:both;
	bottom: 0px;
	text-align:center;
	height: 50px;
	width: 100%;
    background: #6cf;
}
 Modifié par kwad (26 Jul 2013 - 15:04)