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chromium/third_party/blink/web_tests/animations/composition/font-weight-composition.html 

<!DOCTYPE html>
<meta charset="UTF-8">
<body>
<script src="../interpolation/resources/interpolation-test.js"></script>
<script>
assertComposition({
  property: 'font-weight',
  underlying: '500',
  addFrom: '100',
  addTo: '500',
}, [
  {at: -0.3, is: '480'},
  {at: 0, is: '600'},
  {at: 0.5, is: '800'},
  {at: 1, is: '1000'},
  {at: 1.5, is: '1000'},
]);

assertComposition({
  property: 'font-weight',
  underlying: '200',
  addFrom: '600',
  replaceTo: '500',
}, [
  {at: -0.3, is: '889.75'},
  {at: 0, is: '800'},
  {at: 0.5, is: '650'},
  {at: 1, is: '500'},
  {at: 1.5, is: '350'},
]);
</script>
</body>