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chromium/third_party/blink/web_tests/external/wpt/intersection-observer/scroll-margin-iframe.html 

<!DOCTYPE html>
<meta name="viewport" content="width=device-width,initial-scale=1">
<link rel="help" href="https://www.w3.org/TR/intersection-observer/#dom-intersectionobserver-scrollmargin">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="./resources/intersection-observer-test-utils.js"></script>

<iframe
  id="target-iframe"
  src="resources/iframe-no-root-subframe.html"
  width="300"
  height="200"
></iframe>

<script>
var iframe = document.getElementById("target-iframe");
var target;
var entries = [];

iframe.onload = function() {
  runTestCycle(function() {
    assert_true(!!iframe, "iframe exists");

    target = iframe.contentDocument.getElementById("target");
    assert_true(!!target, "Target element exists.");

    var observer = new IntersectionObserver(function(changes) {
      entries = entries.concat(changes)
    }, {
      scrollMargin: "18px"
    });
    observer.observe(target);

    entries = entries.concat(observer.takeRecords());
    assert_equals(entries.length, 0, "No initial notifications.");

    runTestCycle(testIntersection, "Test scroll margin intersection");
  }, "Observer with the implicit root; target in a same-origin iframe.");
};

function testIntersection() {
  assert_equals(entries.length, 1, "IntersectionObserverEntryCount");
  assert_true(entries[0].isIntersecting, "isIntersecting");
  assert_approx_equals(entries[0].intersectionRatio, 0.1, 0.001, "intersectionRatio");
}
</script>