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<!DOCTYPE html>
<html>
<head>
<style>
#background {
background-color: yellow;
display: inline-block;
}
.container {
font-size: 0px;
}
</style>
</head>
<body>
<div id="background">
<div class="container">
<canvas id ="opaque" width="100" height="100"></canvas>
</div>
<div class="container">
<canvas id ="nonopaque" width="100" height="100"></canvas>
</div>
</div>
<script>
function drawCanvas(canvasID, hasAlpha) {
var canvas = document.getElementById(canvasID);
var context = canvas.getContext("2d", {alpha: hasAlpha});
context.clearRect(0, 0, canvas.width, canvas.height);
context.fillStyle = 'blue';
context.fillRect(20, 20, 60, 60);
};
drawCanvas('opaque', false);
drawCanvas('nonopaque', true);
</script>
</body>
</html>