CSS
#fixed {
position:fixed;
right:0;
float: right;
width: 100px;
background: rgba(255,0,0,0.7);
height:400px;
text-align: center;
}
#relative {
position:relative;
margin-right: 100px;
background: rgba(0,0,255,0.7);
height:400px;
width:100%;
display: inline-block;
float:right;
text-align: center;
}
HTML
<div id="fixed" >
I am fixed.
</div>
<div id="relative">
I am relative, fluid, width 100%
</div>