File: background-image-alpha-ref.html

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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>