Postiion by css

demo for stackoverflow

HTML

<div class="header" id="one">hi</div>
<div class="border" id="two">	
    <canvas></canvas>
</div>

CSS

body {
    background:red;
    color: #FFFFFF;
}

.header{
    float: left;
    background: black;
}

canvas{
    height:100%;
}

.border{
    position: absolute;
    z-index:0;
	top:120px;
	bottom:0;
    padding:0;
    margin:0;
}