#header{
    height: 100px;
    width: 100%;
    background-color: gray;
    display: inline-block;
}

.left{
    height: 100%;
    width: 100px;
    background-color: yellow;
    float: left;
}

#footer{
    height: 100px;
    width: 100%;
    background-color: gray;
    display: block;
    clear: both;
}