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chromium/third_party/blink/web_tests/paint/invalidation/resources/fixed-iframe-content.html 

<!DOCTYPE html>
<div id="div" style="width: 100px; height: 100px; background-color: red"></div>
<script>
// For manual testing in browser when cross frame scripting is not allowed.
if (!window.testRunner) {
  setTimeout(function() {
    document.getElementById('div').style.backgroundColor = 'green';
  }, 500);
}
</script>