body{
    position: relative;
}
body,html{
    width: 100%;
    height: 100%;
}
header{
    height: 100px;
    width: 100%;
    background-color: #f00;
}
nav{
    position: absolute;
    width: 100px;
    height: 100%;
    background-color: #000;
    left: 0;
    top:0;
    padding-top: 100px;
}
section{
    width: 100%;
    height: 100%;
    padding-left: 100px;
}