<style>
    .green-background {
        width: 400px;
        height: 400px;
        background: -webkit-canvas(large-canvas) no-repeat 50% 50%;
	}
</style>
<body>
    <div class="green-background"></div>
    <script>
        var ctx = document.getCSSCanvasContext('2d', 'large-canvas', 400, 400);
        ctx.fillStyle = 'green';
        ctx.fillRect(0, 0, 400, 400);
    </script>
</body>