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chromium/third_party/blink/web_tests/fast/canvas/canvas-clearRect-first.html 

<html>
<body>
<canvas id="c" width=100 height=100></canvas>
<script>

// Test Canvas content shows up correctly after a full repaint.
var c2 = document.getElementById("c");
var ctx = c2.getContext('2d');

// Since it checks if content retaining |mode_| changes from frame to frame,
// multiple calls to rAF are used to assure the craetion of new frame.
ctx.fillRect(5, 5, 50, 50);
ctx.clearRect(0, 0, 800, 1000);
window.requestAnimationFrame( dt=> {
  ctx.fillRect(50, 50, 50, 50);
  ctx.clearRect(50, 50, 50, 40);
  window.requestAnimationFrame(dt => {
    ctx.fillRect(0, 0, 15, 15);
  });
});

</script>
</body>
</html>