style> 
#yellow{ width:400px; border:1px solid #FF9900; background:#FFCC99; float:left;} 
#blue{ width:300px; height:100px; border:1px solid #0066FF; background:#00CCFF; float:left;} 
#red{ width:100px; height:100px; border:1px solid #FF3300; background:#FF9900; float:right;} 
/style> 

400px 
div id="yellow"> 
 div id="blue">300px/div> 
 div id="red">100px/div> 
/div>