mozilla-release/testing/web-platform/tests/element-timing/resources/iframe-with-square-sends-entry.html

<!DOCType html>
<html>
<style>
body {
  margin: 0;
}
</style>
<body>
<script>
  const observer = new PerformanceObserver(entryList => {
    top.postMessage({
      'length' : entryList.getEntries().length,
      'entryType' : entryList.getEntries()[0].entryType,
      'rect' : entryList.getEntries()[0].intersectionRect,
      'naturalWidth' : entryList.getEntries()[0].naturalWidth,
      'naturalHeight' : entryList.getEntries()[0].naturalHeight,
      'id': entryList.getEntries()[0].id,
      // Elements cannot be cloned, so just send the element ID.
      'elementId' : entryList.getEntries()[0].element.id,
    }, '*');
  });
  observer.observe({entryTypes: ['element']});
</script>
<img src=square100.png id=iframe_img_id elementtiming=my_image/>
</body>
</html>