third_party/web_platform_tests/intersection-observer/shadow-content.html

<!DOCTYPE html>
<meta name="viewport" content="width=device-width,initial-scale=1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="./resources/intersection-observer-test-utils.js"></script>

<style>
pre, #log {
  position: absolute;
  top: 0;
  left: 200px;
}
</style>

<div id="host"></div>

<script>
var vw = document.documentElement.clientWidth;
var vh = document.documentElement.clientHeight;

var entries = [];
var target;

runTestCycle(function() {
  var shadowHost = document.getElementById("host");
  assert_true(!!shadowHost, "Host exists");
  var shadowRoot = shadowHost.attachShadow({ mode: "open" });
  assert_true(!!shadowRoot, "Shadow root exists");
  shadowRoot.innerHTML = "<div id='target' style='width: 100px; height: 100px; background-color: green;'></div>";
  target = shadowRoot.getElementById("target");
  assert_true(!!target, "target exists");

  var observer = new IntersectionObserver(function(changes) {
    entries = entries.concat(changes)
  });
  observer.observe(target);
  entries = entries.concat(observer.takeRecords());
  assert_equals(entries.length, 0, "No initial notifications.");
  runTestCycle(step0, "First rAF after creating shadow DOM.");
}, "Observing a target inside shadow DOM.");

function step0() {
  checkLastEntry(entries, 0, [8, 108, 8, 108, 8, 108, 8, 108, 0, vw, 0, vh, true]);
}
</script>