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chromium/third_party/blink/renderer/core/paint/test_data/opacity-animation.html 

<!DOCTYPE html>
<style>
div {
  width: 100px;
  height: 100px;
  background-color: red;
}
.animate {
  animation-name: test;
  animation-duration: 4s;
}
@keyframes test {
  from { opacity: 0.0;}
  to { opacity: 1.0;}
}
</style>
<div id="target" class="animate"></div>;