<!DOCTYPE html>
<style>
#container {
    width: 400px;
}

.square {
    width: 100px;
    height: 100px;
    background-color: blue;
}

#float {
    float: left;
}

#bfc {
    margin-left: 30%;
    overflow: hidden;
}
</style>
<p>You should see two blue squares below.</p>
<div id="container">
    <div id="float" class="square"></div>
    <div id="bfc" class="square"></div>
</div>