<html>
<head>
<style>
canvas {
position: absolute;
top: 100px;
left: 100px;
width: 300px;
height: 300px;
}
div.inner {
position: absolute;
top: 200px;
left: 200px;
width: 100px;
height: 100px;
opacity: 0.5;
background-color: #ffffff;
}
</style>
</head>
<body>
<canvas id="c" height="300px" width="300px"></canvas>
<script>
var c = document.getElementById('c');
var ctxt = c.getContext('2d');
ctxt.fillStyle = 'rgb(0,255,0)';
ctxt.fillRect(0, 0, 300, 300);
</script>
<div class="inner">
</div>
</body>
</html>