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chromium/third_party/blink/web_tests/external/wpt/html/canvas/element/manual/filters/canvas-globalAlpha.html 

<!DOCTYPE html>
<link rel="match" href="canvas-opacity-expected.html"/>
<meta name="fuzzy" content="maxDifference=0-2; totalPixels=0-14000">
<body>
  <canvas id="canvas" width="300" height="300"></canvas>
</body>
<script>
  var canvas = document.getElementById('canvas');
  var ctx = canvas.getContext('2d');
  ctx.globalAlpha = 0.75;
  ctx.fillStyle = 'teal';
  ctx.fillRect(10, 10, 50, 50);
  ctx.globalAlpha = 0.5;
  ctx.fillStyle = 'magenta';
  ctx.fillRect(70, 10, 50, 50);
  ctx.globalAlpha = 0.25;
  ctx.fillStyle = 'orange';
  ctx.fillRect(10, 70, 50, 50);
  ctx.globalAlpha = 0;
  ctx.fillStyle = 'chocolate';
  ctx.fillRect(70, 70, 50, 50);

  ctx.globalAlpha = 0.8;
  ctx.fillStyle = 'cyan';
  ctx.fillRect(10, 150, 50, 50);
  ctx.globalAlpha = 0.6;
  ctx.fillStyle = 'red';
  ctx.fillRect(20, 150, 50, 50);
  ctx.globalAlpha = 0.4;
  ctx.fillStyle = 'yellow';
  ctx.fillRect(30, 150, 50, 50);
  ctx.globalAlpha = 0.2;
  ctx.fillStyle = 'green';
  ctx.fillRect(40, 150, 50, 50);
</script>