This won't work. You will just make a 90px gap between the article div & sidebar div.
Originally Posted by slaughters
Do this instead using a container div:
You can use pixels instead of percent for the widths but ensure the container div is wide enough for both widths side-by-side. Adding padding/margins to the divs will reduce the amount of width available.
<div id="article" style="float:left; width:75%"></div>
<div id="sidebar" style="float:right; width:25%"></div>