third_party/web_platform_tests/intersection-observer/resources/v2-subframe.html

<!DOCTYPE html>
<div id="target">target</div>
<script>
var delay = 100;
var results = [];

function waitForNotification(f) {
  setTimeout(() => {
    requestAnimationFrame(function () {
      requestAnimationFrame(function () {
        setTimeout(f)
      })
    })
  }, delay)
}

window.addEventListener("message", event => {
  waitForNotification(() => {
    window.parent.postMessage(results.map(e => e.isVisible), "*");
    results = [];
  });
});

onload = () => {
  var target = document.getElementById("target");
  var observer = new IntersectionObserver(entries => {
    results = entries;
  }, {trackVisibility: true, delay: delay});
  observer.observe(document.getElementById("target"));
  window.parent.postMessage("", "*");
};
</script>