Edit in JSFiddle

<div id="sidebar">
    I'm a sidebar!
</div>
<div id="content">
    Full-screen sidebar? Have no fear!
    Absolute positioning to the rescue!
</div>

#sidebar, #content {
    position: absolute;
    top: 0;
    bottom: 0;
}
#sidebar { left: 0; width: 10em; background: lightgray; }
#content { left: 10em; right: 0; }