Let's say we have this very simple scenario
 <div class="content">
    <div class="left">
       <p>some content</p>
    </div>
    <div class="right">
       <p>some content</p>
    </div>
 </div>
 <div class="content">
    <div class="left">
       <p>some content</p>
    </div>
    <div class="right">
       <p>some content</p>
    </div>
 </div>
And this is the styling:
 .content {
    width: 960px;
    height: auto;
    margin: 0 auto;
    background: red;
    clear: both;
 }
 .left {
     float: left;
     height: 300px;
     background: green;
 }
 .right {
     float: right;
     background: yellow;
 }
And the thing is... when I add content to <div class="right"> it should pull down the parent div, and we need to see the red background... the thing is, I cannot see the red background fill all the height.
EDIT: here is a fiddle to test
 
     
     
     
     
     
     
    