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chromium/third_party/blink/web_tests/external/wpt/web-animations/responsive/resources/block.html 

<!DOCTYPE html>
<style>
  .testBlock {
    width: 100px;
    height: 100px;
    background: #00cc66;
    border-top: 50px solid #ffff00;
  }
</style>
<div class="testBlock" id="testBlock"></div>