I guess your elements on the page messes up because you don't clear out your floats, check this out
HTML
<div class="wrap">
<div class="floatleft"></div>
<div class="floatright"></div>
<div style="clear: both;"></div>
</div>
CSS
.wrap {
width: 100%;
}
.floatleft {
float:left;
width: 80%;
background-color: #ff0000;
height: 400px;
}
.floatright {
float: right;
background-color: #00ff00;
height: 400px;
width: 20%;
}