Percentages are more flexible because they can scale to the viewport.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title></title> <style type="text/css"> .container{ width: 800px; height: 800px; background: pink; } * .box { float: right; overflow: auto; visibility: visible; width: auto; height: 100px; margin: 10px; padding: 10px; background: red; } * .small { float: left; right: 200px; height: 50%; background: yellow; } </style> </head> <body> <div class="container"> <div class="small">this is a test. <BR/>this is a test. this is a test. this is a test. </div> <div class="box">this is a test</div> </div> </body> </html>