Question

How can i get div B to have a width of 100% while nested in Div A which has a fixed width?

.divA{width:600px; margin:0 auto;}
.divB{width:100%; height:20px;  background-color:black}

.filler{height:800px; background-color:grey}

<div class='divA'>
 <div class='filler'></div>
 <div class='divB'></div>
</div>

edit the problem was: .divA had a position:relative i had to take out

Answer 1

use

.divB {
    width:100%;
    height:20px;
    min-width:600px;
    background-color:black;
    position:absolute;
    left:0;
    right:0;

 }

FullScren view

Fiddle