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chromium/third_party/blink/web_tests/paint/invalidation/invalidations-in-iframe.html 

<!DOCTYPE html>
<style>
  iframe {
    display: block;
    border: 0;
  }
</style>
<script src="resources/text-based-repaint.js"></script>
<script>
// Tests that invalidations are generated for composited sub-frames.
function repaintTest() {
  document.querySelector("iframe").contentDocument.querySelector("div").style.backgroundColor = "#88f";
  document.querySelector("div").style.backgroundColor = "#8f8";
}
window.addEventListener("load", runRepaintTest);
</script>
<iframe srcdoc="&lt;div style='will-change:transform;width:100px;height:100px;'&gt;&lt;/div&gt;"></iframe>
<div style="width:200px; height:200px"></div>