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<!DOCTYPE html>
<html>
<head>
<title>HTML5 canvas transparent rectangles</title>
<script>
    function draw() {
        var canvas = document.getElementById('myCanvas');
        var ctx = canvas.getContext('2d');

        ctx.fillStyle ="green";
		ctx.globalAlpha=0.1;
		ctx.fillRect(0, 0, 1024, 768);
        for (var i = 1; i <= 10; i++) {
			ctx.fillStyle ="black";
            var alpha = i * 0.1;
            ctx.globalAlpha = alpha;

            ctx.fillRect(50*i, 20, 40, 40);
        }

    }
</script>


<style>
	body{
		background-color: brown;
	}

</style>
</head>

<body onload="draw();" >
    <canvas id="myCanvas" width="550" height="200">
    </canvas>
</body>
</html>