responsive web design

HTML

<div id="nav">
    <div class="block" id="left">
        <div>
            <h1>blah blah</h1>
        </div>
    </div>
    <div class="block" id="middle">
        <div>
            <h1>blah blah</h1>
        </div>
    </div>
    <div class="block" id="right">
        <div>
            <h1>blah blah</h1>
            
        </div>
    </div>
</div>

CSS

.block {
    width:33%;
    height:50%;
    position:relative;
    float:left;
    background-color:#CCC;
}

.block>div {
    margin:20px;
}