Solution #3. To apply overflow property.

We will apply overflow: auto to the div.container container block. The height of container repeated the height of floating blocks.

<div class="container">
    <div class="left">
        ...
    </div>
    <div class="right">
        ...
    </div>
    ...
</div>

.left
{
border: 2px solid #33F;
float: left;
}

.right
{
border: 2px solid #F00;
float: right;
}

div.container {
    background-color: #CCC;
    border: 1px dashed #333;
    overflow: auto;
}