Démo html 5

header { height:80px; background-color:#999; margin:0 0 10px 0; }

nav { height:10px; background-color:#999; margin:0 0 10px 0; }

section {
float:left;
min-height:300px;
width:450px;
background-color:#999;
}

aside {
float:left;
min-height:300px;
width:140px;
background:#999;
margin: 0 0 10px 10px; }

footer { height:30px; background-color:#999; margin:0 0 10px 0; clear:left; }