removed old "body and header_nav" for...

added 

body {
    height:1000px;
    width:100%;
    background-color:#eee;
}
.header {
    width:100%;
    height:100px;
    background: #26b;
    color: #fff;
    position:fixed;
    top:0;
    left:0;
    transition: height 500ms, background 500ms;
}
.header.tiny {
    height:40px;
    background: #aaa;
}