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chromium/third_party/blink/web_tests/fast/canvas/OffscreenCanvas-text-FontFace.html 

<!DOCTYPE html>
<html>
<body>
<canvas id='output' width='400' height='400'></canvas>
<script>
if (window.testRunner) {
  testRunner.waitUntilDone();
}

let ofc = new OffscreenCanvas(100, 100);
let ctx = ofc.getContext("2d");

draw();
requestAnimationFrame(() => {
  requestAnimationFrame(() => {
    draw();
    if (window.testRunner) {
      testRunner.notifyDone();
    }
  });
});

function draw() {
  ctx.clearRect(0, 0, 100, 100);
  ctx.font = "50px Ahem";
  ctx.textBaseline = "top";
  ctx.fillStyle = "red";
  ctx.fillText("X", 0, 0);

  var c = document.getElementById("output").getContext("2d");
  c.drawImage(ofc, 0, 0);
}

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